Steps to reproduce the issue

1)

• I installed Joomla! choosing the Multilingual option choosing ITALIAN and ENGLISH-GB
• I have enabled the plugin "System - Language Filter setting" the "Removal of the default language prefix" and "Language x-default" in Italian
• I have enabled the "System - Language Code" plugin leaving the default settings
• I have enabled from the Joomla! Settings "Rewrite SEF URL" and commented #Options +FollowSymlinks and #Options -Indexes
• On the Item Options I have enabled "Modern URL Routing" and "Remove ID from URLs"

2)

• I created one tag by calling it "[INGREDIENTI]" by setting the language to "All"
• I created 3 tags calling them "Cipolla", "Prosciutto" and "Polipo" and set for each the main tag "[INGREDIENTI]" created previously and set the language "All"

• I created the tag "[RICETTE]" by setting the language to "All"
• I created the "Antipasti" tag by setting the main tag to "[RICETTE]" and language to "All"
• I created the "Antipasti vegetariani" tag by setting the main tag to "Antipasti" and the language to "All"
• I created the "Antipasti di carne" tag by setting the main tag to "Antipasti" and the language to "All"
• I created the "Antipasti di pesce" tag by setting the main tag to "Antipasti" and the language to "All"

3)

• I created two Categories calling them "Ricette" setting the Italian language and "Recipes" setting the English language. I associated them with each other
• I created the "Antipasti" sub-category in "Ricette" by setting the Italian language and giving the tags "Antipasti", "Antipasti vegetariani", "Antipasti di carne" and "Antipasti di pesce"
• I created the "Appetizers" sub-category in "Recipes" by setting the english language and giving the tags "Antipasti", "Antipasti vegetariani", "Antipasti di carne", "Antipasti di pesce". I also associated it with the "Antipasti" category.

4)

• I created an article by calling it "Antipasto A" by setting the Italian language, the "Antipasti" category and giving the tags "Antipasti vegetariani" and "Cipolla"

• I created an article by calling it "Antipasto B" by setting the Italian language, the "Antipasti" category and giving the tags "Antipasti di carne" and "Prosciutto"

• I created an article by calling it "Antipasto C" by setting the Italian language, the "Antipasti" category and giving the tags "Antipasti di pesce" and "Polipo"

• I created an article by calling it "Appetizer A" by setting the english language, the "Appetizers" category and giving the tags "Antipasti vegetariani" and "Cipolla" associating it with "Antipasto A"

• I created an article by calling it "Appetizer B" by setting the english language, the "Appetizers" category and giving the tags "Antipasti di carne" and "Prosciutto" associating it with "Antipasto B"

• I created an article by calling it "Appetizer C" by setting the english language, the "Appetizers" category and giving the tags "Antipasti di pesce" and "Polipo" associating it with "Antipasto C"

5)

• I have 3 menus, "Main Menu", "Main Menu (en-GB)" and "Main Menu (it-IT)" and each has its own Home Page.

• I created the "Antipasti" menu item in the "Main Menu (it-IT)" by setting "Blog Category", choosing the "Antipasti" category by setting the italian language

• I created the "Appetizers" menu item in the "Main Menu (en-EN)" by setting "Blog Category", choosing the "Appetizers" category by setting the english language, associating it with the "Antipasti" menu item

• I created the menu item "Antipasti vegetariani" under the "Antipasti" item by setting the category "Ricette", the tag "Antipasti vegetariani", italian language

• I created the menu item "Antipasti di carne" under the "Antipasti" item by setting the category "Ricette", the tag "Antipasti di carne", italian language

• I created the "Antipasti di pesce" menu item under the "Antipasti" item by setting the category "Ricette", the tag "Antipasti di pesce", italian language

• I created the menu item "Vegetarian appetizers" under the item "Appetizers" by setting the category "Recipes", the tag "Antipasti vegetariani", english language, associated with "Antipasti vegetariani"

• I created the "Meat appetizers" menu item under "Appetizers" by setting the "Recipes" category, the tag "Antipasti di carne", english language, associated with "Antipasti di carne"

• I created the menu item "Fish appetizers" under the "Appetizers" item by setting the category "Recipes", the tag "Antipasti di pesce", english language, associated with "Antipasti di pesce"

At this point the links of the articles will be:

/antipasti/antipasto-a
/en/appetizers/appetizer-a

/antipasti/antipasto-b
/en/appetizers/appetizer-b

/antipasti/antipasto-c
/en/appetizers/appetizer-c

6)

• Now I create two new Main Menus by calling them "Ingredienti (it-IT)" and "Ingredients (en-EN)"

• Inside "Ingredienti (it-IT)" I create the menu item "Cipolla" by selecting the type "Blog Category", choosing the category "Ricette", the tag "Cipolla", italian language (on Options link I select "hidden")

• Inside "Ingredienti (it-IT)" I create the menu item "Prosciutto" by selecting the type "Category Blog", choosing the category "Ricette", the tag "Prosciutto", Italian language (on Options link I select "hidden")

• Inside "Ingredienti (it-IT)" I create the menu item "Polipo" by selecting the type "Blog Category", choosing the category "Ricette", the tag "Polipo", Italian language (on Options link I select "hidden")

• Inside "Ingredients (en-EN)" I create the menu item "Onion" by selecting the type "Blog Category", choosing the category "Recipes", the tag "Cipolla", english language, associating the entry with "Cipolla" (on Link options select "hidden")

• Inside "Ingredients (en-EN)" I create the menu item "Ham" by selecting the type "Blog Category", choosing the category "Recipes", the tag "Prosciutto", english language, associating the item with "Prosciutto" (on Link options select "hidden")

• Inside "Ingredients (en-EN)" I create the menu item "Octopus" by selecting the type "Blog Category", choosing the category "Recipes", the tag "Polipo", english language, associating the entry with "Polipo" (on Link options select "hidden")

At this point the links to the public of the "Ingredients" menus will be:

/cipolla
/en/onion

/prosciutto
/en/ham

/polipo
/en/octopus

Expected result

• If I log on "/cipolla" the link works. If I press on the english flag I end up on "/en/onion"
• If I log on "/prosciutto" the link works. If I press on the english flag I end up on "/en/ham"
• If I log on "/polipo" the link works. If I press on the english flag I end up on "/en/octopus"
• If I log on "/onion" the link works. If I press on the italian flag I end up on "/en/cipolla"
• If I log on "/ham" the link works. If I press on the italian flag I end up on "/en/prosciutto"
• If I log on "/octopus" the link works. If I press on the italian flag I end up on "/en/polipo"

Actual result

• If I log on "/cipolla" the link works. If I press on the english flag I end up on "/en/appetizers/fish-appetizers"
• If I log on "/prosciutto" the link works. If I press on the english flag I end up on "/en/appetizers/fish-appetizers"
• If I log on "/polipo" the link works. If I press on the english flag I end up on "/en/appetizers/fish-appetizers"
• If I log in to "/en/onion" the link works. If I press on the italian flag I end up on "/antipasti/antipasti-di-pesce"
• If I log in to "/en/ham" the link works. If I press on the italian flag I end up on "/antipasti/antipasti-di-pesce"
• If I log in to "/en/octopus" the link works. If I press on the italian flag I end up on "/antipasti/antipasti-di-pesce"

System information

• Joomla! 3.9.16
• Php 7.3.13
• MySqli 5.5.62-38.14-log

Additional comments

• If I try to disable some menu items from "Ingredients (it-IT)" and "Ingredients (en-EN)" by pressing on the flags the associated link changes but it is always wrong.