I reported that to apple, but it seems they haven't fix it yet.
I can easily repeat the bug in apps like Pages or Numbers.
However, like icezimy wrote, i think i finally found a workaround! We are testing that in real work...
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I found the issue!
I had an old artstudio pro beta app that expired. Once i deleted the beta app, all of the thumbnails began to appear once again
Is the smudge tool issue being addressed by the next update ?
Thank you both for the replies and suggestions, I’ll try saving my files outside of iCloud as well as manually saving to mitigate issues until Apple hopefully fixes this