We have an old site.
Now we making it multilanguage.
The site has many menu items.
We created new menus for each language and copied all menu items with changing language in batch mode.
Then we added language associations for each menu item.
Ok, for example now we are in the Android category in Spanish language.
URL is site.com/es/android
Joomla's language module has links for all other languages with the same prefix site.com/es/android

At the same time, Falang language module displays CORRECT urls with right prefixes.

But, when we are adding NEW menu items with associations, Joomla's language module displays correct links with correct prefixes.

We tried to resave old menu items - no results.

So, we have no idea why it happens and how to fix it.