Hi,
We discussed a very similar problem in 2017-2018 already: deactivating OneDrive's selective sync worked fine... until a couple days ago. At least for me as, once again, all my thumbnails have vanished, replaced by the generic Photo/Designer icon.
Reactivating/Deactivating Selective Sync did not work.
But moving all my affinity files out of OneDrive and all thumbnails reappears.
And manually duplicating each file and save it back to OneDrive worked too: but it's not a viable solution: open one file at a time in Affinity, copy all its layers, create a new empty file, paste all layers, save it with a new name.
If anybody has a workaround/incantation/whatever, please share it with us. I insist that, at least for me, deactivating OneDrive's Selective Sync does not work any more. I even tried to reinstall Windows to no avail.
I get it it's probably OneDrive's and iCloud (as I have the same problem when using iCloud on Windows). But this problem only occurs with Affinity files, there must be something with the file format itself...
And to be frank it's a real pain to browse a catalog with no thumbnails