It's allegedly a result of how Calibre manages tags in order to display covers but still feels deeply wrong.
Long thread here: https://www.mobileread.com/forums/showthread.php?t=340936
There are two main tag values for Kindle books: EBOK and PDOC.
EBOK is intended for store content. (ebooks) PDOC is intended for user content. (personal documents)
In the past, only EBOK got cover thumbnails. So Calibre tagged books as ebok to get cover thumbnails.
This changed in a relatively recent firmware update for 7th gen and later Kindles, so now PDOC tagged books get cover thumbnails too. (Sept 2022)
I assume that Amazon just looks at the EBOK tag, assumes its an expired loan or something, and deletes it. Since that thread says only EBOKs get deleted. Calibre still tags as EBOK by default. A possible solution is to tag as PDOC instead. That thread mentions the problem started in 7/2021
I didn't change any default settings in either.