Languages URLs not correct

This is all very strange. I can't figure out the cause of the problem.
I disabled ALL modules and plugins on the problematic site, but it didn't help.
I copied the router.php file from FC version 4.2.1, and the site stopped working.
I installed a clean Joomla 5 and FlexiContent master, and everything works.
I copied the entire folders from the problematic site to a clean Joomla and FC master, which is essentially a downgrade:
/administrator/components/com_flexicontent
/components/com_flexicontent
/plugins/flexicontent
/plugins/flexicontent_fields
Thus, the clean site with Joomla 5.3.1 and FC Master now has files from FC 4.2.1. Everything works fine.
I spent several hours on this.
But when I updated the site to the FC Master version, the problem recurred!!!
Then I restored the files from the Master version, and the problem reappeared.
Next, I used the Joomla installer to install version 4.2.1.
The URLs are correct.
Finally, I used the Joomla installer to install the Master version.
The problem reappeared.
Conclusion:
1. When we install a clean Joomla and FC Master, there are no problems.
2. When we update the existing FC 4.2.1 to the Master version, there is a problem.
3. When we simply replace the FC 4.2.1 files with the files from the Master version via FTP, there is no problem.

And the final confirmation that the problem is not in the settings, third-party plugins, modules, or extensions.

I performed a clean installation of Joomla 5.3.1 and FC 4.2.1 again.
I created 3 site languages, menu items, and one article in three languages.
Everything works fine.

Now I am updating FC to the Master version.
And now all alternative links have the current language prefix.
Please note that this is a completely clean installation of Joomla and FC, without any third-party extensions.
The problem only occurs when updating from FC 4.2.1 to the Master version.
Probably, during the update, some files are not updated and remain from the old version.

Watch the video to see how this happens: directory.audio/images/monopics/2025-06-25_15-05-18.mp4

P.S. Hmmm... I think I found something.
You showed me that everything is fine on your demo - flexidemo.com3elles.com/jobboard/demoj4/...offers/gardener.html

Go to the System - Language Filter plugin settings and enable the Remove URL Language Code option. This will remove the default language prefix and you will see this problem on your website.
 

I made a commit 5 days ago
See it here:
github.com/FLEXIcontent/flexicontent-cck...f6c28c3082d3e2e982e3

Did you try with master branch after the above commit ?

Hello Georgios.
I just downloaded and installed FC from Master branch.
No results. Did not help. Alternate URLs still use current language prefix and aliases from current item..

BUT!!! I replaced association.php with same file from FC 4.2.1 - and URLs are ok.
So, problem exact in this file.

Everything is fine. The latest FC Master update solved the problem.
Thank you.

thanks for return