List of steps to reproduce (step by step, including full links if applicable):
Issue 1:

• Open Special:AllMessages ( https://www.mediawiki.org/wiki/Special:AllMessages?prefix=nstab&filter=all&lang=en&limit=5000 for example), try to find "Nstab-project_talk".
• Open Special:AllMessages ( https://www.mediawiki.org/wiki/Special:AllMessages?prefix=nstab&filter=all&lang=en&limit=5000 for example), look at the alleged content of "Nstab-talk".

Issue 2:

• Open MediaWiki:Nstab-talk ( https://www.mediawiki.org/wiki/MediaWiki:Nstab-talk for example), try to look up the default content of this system message.

What happens?:
Issue 1:

• Nstab-project_talk isn't listed.
• Nstab-talk is displayed with an empty default value instead of "Talk".

Issue 2:

• Page appears empty, not matching the actual default message content which is "Talk".

What should have happened instead?:
Issue 1:

• Nstab-project_talk is listed properly.
• Nstab-talk is listed properly.

Issue 2:

• Page shows the text "Talk" even when non-existent.
• Editing the page prefills the form with "Talk".

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:

• Issue is present in MediaWiki 1.35.5
• Issue is present in MediaWiki 1.39.0-wmf.4