I also noticed this issue recently and looked into it a bit... I do believe that something behind the scenes has changed.
First of all, what strikes me as weird, is that the items in your Library are generally read from the Tray files. The Tray files also store the original thumbnails. However, it seems that the game will no longer use them and instead generate new thumbnails which are stored in and read from the localthumbcache.package. I don't really understand why. If you check your households via the Sims 4 Tray Importer, it will also display the original thumbnails from the Tray files. And if you upload the household to the Gallery from the Library, it will also have the original thumbnails from the Tray files. So it only seems to be an issue in the Library, where the game will try to retrieve the thumbnails from the localthumbcache.package.
Consequently, it seems that deleting the localthumbcache.package will trigger the regeneration of thumbnails in the Library. I'm not sure if this was changed on purpose, I can't figure the reasoning behind it as it slows down the game. But if this is how it works now, we will have this issue repeatedly, since deleting this localthumbcache.package is necessary from time to time.
I also run a test where I let the game generate all thumbnails in my Library, then added a custom pose pack and one of my mods for Gallery poses and restarted the game to see if this is also a trigger. But the thumbnails stayed. So I think this is not a mod issue and not an issue with custom poses.
Currently there is also another issue that started happening affecting thumbnails in-game (for example the thumbnails in the relationship panel). It seems that in this case, the game will regenerate these thumbnails each time you restart the game, even when the thumbnails already exist in the localthumbcache.package, thus unnecessarily bloating up this file and also slowing down the game. More info on it here: https://answers.ea.com/t5/Bug-Reports/OPEN-Thumbnails-regenerate-with-every-session-localthumbcache/m-p/13911023#M284866
The library issue started happening a bit earlier for me I believe. But I'm wondering if all those issues are related somehow.