For whatever reason, it seems that the project name needs to be repeated in the hashtag field. Perhaps the "additional hashtags" field would be more appropriately labeled as simply "hashtags"

I think this is likely an upstream problem. I think we try to add the "natural" hashtag on a rename, but maybe we're looking at the old "hashtags" field, seeing the "natural" hashtag is already there, and not realizing that it's being removed by the edit. Let me see if I can reproduce/understand this.

There may also be a secondary issue where everything works fine, except the "Save" button redirects you to the old object URI (from before your edit), which your edit may have changed. This is an issue in one interface in Diffusion today. The description above doesn't sound exactly like the same issue, but there might be something roughly similar going on.

Perhaps the "additional hashtags" field would be more appropriately labeled as simply "hashtags".

The intent, at least, is that the "natural" hashtag gets managed automatically and you don't need to list it explicitly, but it definitely sounds like that's not working correctly in this example.

MediaViewer example works again. Changelog: https://phabricator.wikimedia.org/project/manage/230/

Same problem now for GrowthExperiments (#growthexperiments). Changelog: https://phabricator.wikimedia.org/project/manage/3716/
The alternative hashtag GrowthExperiments (#mediawiki-extensions-growthexperiments) seems to create a link at least.

Still happens, see T239038 in which https://phabricator.wikimedia.org/tag/user-andreas/ was created and renamed to https://phabricator.wikimedia.org/tag/user-apap04/

On https://phabricator.wikimedia.org/project/edit/4396/ ,

1. I renamed back from #User-Apap04 to #User-Andreas, https://phabricator.wikimedia.org/tag/user-andreas/ still did not work.
2. I removed the "Additional hashtag" #User-Apap04, https://phabricator.wikimedia.org/tag/user-andreas/ still did not work.
3. I added the "Additional hashtag" user-andreas(in small letters). https://phabricator.wikimedia.org/tag/user-andreas/ works.

Upstreamed as https://we.phorge.it/T15636 .
Simple fix / workaround: Access project via its ID, explicitly add the current primary slug under "Additional Hashtag".