I deleted the app using cleanmymac, reinstalled, and the same behavior happened.
Moved the library off iCloud to Documents and the same happened.
I think it could be either an M1 thing since I haven't tried the x64 version , or a bug with the spellchecker.
It only happens once in a while, but once it happens it keeps recurring even if you move to a different document.
I've been able to replicate the behavior by using the spellcheck a lot. So I think that it's likely that it happens after using the spellcheck for a certain amount of times, which explains why it only happened before once every a few days.
I’m sorry that I don’t have an answer for you currently. I’ll keep digging in to it, but I haven’t found a cause or solution for you yet.
I’d be curious to know if it happens with the x64 version, but I’m guessing it will, and whether it occurs on other machines if you have any available to test with.