Apple Photos indexing issues

Here are some troubleshooting things to try:

• Restart the computer (As you've tired, of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. So you'll need to connect the new user to your Apple account and you would need to move your Library out of your own user's folder up to the general "Users" folder. 
• Rebuild your Photos Library, again--close Photos and re-launch by option-command-clicking the app icon. Since you're using iCloud Photos, you might want to turn off iCloud, perhaps turning off wifi, itself. When you reconnect, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

Where is your library stored? If it's on an external disk that must be formatted Mac OS X Extended, Journaled or apfs, and directly attached to your Mac, not on a NAS or other networked location.

So, in other words, you don't have a back up. Therefore, I'm afraid the route out of this is not easy, and may involve dataloss.

Create a new library and use PowerPhotos

https://www.fatcatsoftware.com/powerphotos/

to copy the images from the damaged library to the new one.

How are you searching your Mac for keywords? Are you using the search field or are you using a smart album? I am getting very different results when using a smart album compared to the search field. A smart album, for example with the rule "keyword is ..." is showing all photos with the keyword, and only the photos with the keyword, showing the indexing is correct, while the search field is showing a selection of items related to the search string. To see, if the indexing is correct, you may want to test with a smart album.

• The smart album "Keyword is Bird" is finding 3038 photos and 13 videos.
• The search for "Bird" is given mixed results - 3059 items including photos with any metadata with "Bird" as a text:

Photos for Mac used to have a specific search result labelled "keyword", that is gone now in macOS 26 Tahoe.

Searching on my iPhone 15 Pro Max for "Bird" ais giving nearly the same result - the differences might be explained by the iCloud syncing, that has been paused.

The iPhone is still including the keywords in the results for a text search, even if it is not showing the keywords. but the search on the Mac is now as limited as in the iPhone; I am very glad the Mac is still supporting keywords in the smart albums.

If it's only been a few days, I'm not sure you should panic. If it's still indexing in 10 more days, then that's time to start worrying. This depends a lot on the size of the Library, of course. But Photos has a lot to do.

That blows that theory. Can I ask: how did you rebuild the library?

Sounds to me like that library is borked beyond repair. Do you have a back up?