As i wrote few times the bug is caused by iOS.
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...

Ok, thanks for info.

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

Chromatic aberration splits the light (color) to three parts: red, green and white. Black is "no light" so it cannot be split.

Yes, it works ok in the beta.

Thanks for reporting that, i will fix it asap

File saving issue you have is probably caused by this system bug i reported to Apple and posted here:
https://discussions.apple.com/thread/254870123

If possible please reply to that post, it will keep it visible on the top and hopefully make Apple fix it.

No, sorry. We cannot put brush setting there, there are too many of them...

Unfortunately it is a system bug in iOS/iPadOS, which can happen in many other apps, like Pages, Numbers etc.
We reported the bug to apple, and send them video with all steps to reproduce it.

We will also soon publish a new update which doesn't fix this but displays an error message and doesn't grey out the top menu.

We hope Apple will fix it soon.

ps. Please reply to this post if possible:
https://discussions.apple.com/thread/254870123
This will increase the chance to be fixed by apple.