Books disappearing from iBooks library

I have over 1000 EPUB books in iBooks. Sometimes, with no apparent rhyme or reason, I could be in the middle of reading a book, the screen flickers, and all the books disappear, except for the iPad user guide. This can happen a few times per day, or once a month. Been going on for a couple of years now, no apparent relationship to sync, or internet connection, or anything else. Unrelated to updates. If I wait long enough (again, no specific timeframe, from a few minutes to few hours), the books re-appear, although all my custom ordering in the collections appears to be messed up. When they appear, it's like they are being re-downloaded from iCloud, takes a while for all the covers to load, etc. However, it appears that some books are lost for good, I see empty collections that I know had books in them before.

Is this a known issue, is there a fix to this?

iPad Mini 4, latest iPadOS.