Where are you seeing the & to & conversion? This is the expected behavior when viewing the page source/markup.

Joomla 3.9.19 & 3.9.20 convert the URL Type string from http://www.aaa.de/aaa/aaa/index.php?option=com_content&view=article&=5&Itemid=120#section1 to http://www.aaa.de/aaa/aaa/index.php?option=com_content&view=article&id=5&Itemid=128#section1&Itemid=120 and that is not only in the source code inspector but also when I hover over the link on my homepage with a mouse.

Why I am using the URL Type? The answer is, to be able to scroll from my submenu link to a specific #anchor in my page. But it doesn't work with the 3.9.19 & 3.9.20 version/

The main issue here seems to be that the fragment #section1 is not at the end of the "converted" URL.

... #section1&Itemid=120

An Example with installed demo data "learning" in Joomla 3.9.21-dev:

• SEO on, URL rewrite on.
• Existing article with ID 5 (Title: "Authentication")
• This article has already a menu item "Authentication" (ID 318). But that's not too relevant here.
• When I create a menu item type URL with link

index.php?option=com_content&view=article&id=5#section1

• The output of Joomla is

/using-joomla/extensions/plugins/authentication#section1&Itemid=615

• The trailing itemid is the id of the newly created URL item type.
• The fragment #section1 is not at the end.

if I enter index.php?option=com_content&view=article&id=5#section1 into the URL field of my menu > menu item, save it and refresh my homepage,

That is your mistake as that is not the url. The url includes the itemid.

Personally I would recommend using the SEF url instead as its much easier.

For example the SEF url is www.example.com/contact

So I make the new menu url /contact#map or www.example.com/contact#map

Works perfectly

Whwn I debug my example item above inside helper.php of mod_menu then I get

[15] => Joomla\CMS\Menu\MenuItem Object
        (
            [id] => 615
            [menutype] => mainmenu
            [title] => URLtesten
            [alias] => url
            [note] => 
            [route] => url
            [link] => index.php?option=com_content&view=article&id=5#section1
            [type] => url
            [level] => 1
            [language] => *
            [browserNav] => 0
            [access] => 1
            [params:protected] => {"menu-anchor_title":"","menu-anchor_css":"","menu-anchor_rel":"","menu_image":"","menu_image_css":"","menu_text":1,"menu_show":1,"pageheader_suffix_ghsvs":"","page_subheading_ghsvs":"","mega-menu-ul-class":"","mega-menu-li-class":""}
            [home] => 0
            [img] =>  
            [template_style_id] => 0
            [component_id] => 0
            [parent_id] => 1
            [component] => 
            [tree] => Array
                (
                    [0] => 615
                )

            [query] => Array
                (
                    [option] => com_content
                    [view] => article
                    [id] => 5#section1
                )

        )

Look at the [query] block. The [id] => 5#section1 looks weird somehow.

it should be

[id] => 5
[anchor] => section1

right?

it should be

[id] => 5
[fragment] => section1

or something like that. I don't know. I always use the recommendation of brianteeman with hidden article menu items if I don't want them in my front end menu.

But the fragment should be kept always at the end of the url if the url is rebuilt; somehow...

It makes no sense at all for you to be trying to build sef urls for this. Just do what I say and use the sefurl in the link

I'll try brianteeman's recommendation with the sefurl.

Closing per #29797 (comment)