Photos are missing but cannot be re-imported
I have a large photo library - over 30K photos. I recently discovered that several thousand photos were missing from 2015.
I thought I just hadn't imported them from lightroom properly when I migrated, so I re-exported from lightroom and tried to re-import them.
When I re-imported them, there weren't any to import. It said they were already imported. But when I manually scroll back to the first date that should have photos (2015-01-02) I see they are in the folder that I imported, but they're not in my photo library.
I've checked the trash (empty), I don't use hidden photos. I've also restored my photo library (cmd+opt+open app). I've restarted photos/computer and of course googled everything I can think of. I've also checked that they're missing on both my mac & my iphone (same icloud account)
Any tips would be appreciated.
MacBook Pro 13″, macOS 12.1