Why do names I have tagged in my photos randomly disappear?

Every couple of weeks, hundreds of face names that I have laboriously tagged in Photos simply disappear. There's no pattern I can discern, no reason I can fathom why some names stick and others vanish. This has happened to me again and again, and it's apparently happened to others who have shared their experience on this forum, but I've never seen any evidence of a fix. I suspect the problem has to do with iCloud when syncing photos across multiple devices, but I can't be sure.

Does anyone have any suggestions? This has been a problem for years. Why can't Apple fix it?

-matt