I checked it on 3 different sites with Drupal versions mentioned below, And t...

I checked it on 3 different sites with Drupal versions mentioned below, And the issue still exists:

• 9.0.5
• 8.9.3
• 8.8.4

Steps in #13 mentioned this being the problem with language detection method of session, But I think its beyond that and is rather true of other language detection methods as well.

Assumption: The default site language is English.

Steps to reproduce:

1. Enable language & content_translation modules
2. Go to the path admin/config/regional/language. Add Spanish & French languages.
3. Go to the path /admin/config/regional/language/detection and ensure your language detection and selection methods for Interface text language detection and Content language detection are set to URL. [I don't this step matters and the issue exists for other detection and selection methods too. I have mentioned this step as I tested the whole thing with this config. See attached screenshot for the config.]
4. Create an content type such that its translatable(See attached screenshot for content type configuration)
5. Add content in English and add URL alias
6. Translate the English node to Spanish such that it also has alias. [This step is optional]
7. Now, Change the language of English node to French. Without doing anything to URL alias, Save the node.
8. Edit the newly saved node and You will notice that the URL Alias is no longer present on it.

Summary: Changing the language of source translation is causing the node to loose its URL alias.