Recreating library to fix missing cover art made a total chaos

Plex Media Server Metadata & Adding Files
1

Server Version#: 1.25.5.5492
Player Version#:4.6.91

Hi,
I have a pretty substantial audiobook library (>2500 titles). All books are single file .m4b, encoded with the same tool (ffmpeg) and tagged correctly using mp3tag.
One day, when adding new titles to that library, Plex stopped recognizing embedded album art, which really bothers me.
I tried every tip I could find on the web to get Plex to display the covers, to no avail.
They get displayed fine in every other app.
As a last resort, I completely removed the library and created a new one. The results are devastating.
Although I checked the “prefer local metadata” box absolutely NO album art is shown.
Even worse, Plex seems to IGNORE metadata deliberately.
Files are appearing under wrong artist names, genres are changed (I picked “embedded metadata” for genre supplier).
What can I do?

2

OK, found out what made about 50% of this mess: Copied over files from macos to my linux server messed up the encoding, so that files containing special characters were displayed without cover art. I fixed that, now most covers are displayed, but not all.

Still, I can’t stop plex from ignoring my (correct) metadata for artist etc. and use something wrong.
One example: I have both books, “Ready Player One” and “Ready Player Two”. Both named and tagged correctly. In Plex I see two times “Ready Player Two”. The embedded cover gets ignored and replaced by something completely unrelated (a portrait of somebody I can only assume is the author?)

Why can’t I configure Plex to simply go with the local data and NOT scrape at all?

P.S. Using plex for about 8 years now, and the problem just got worse over the last months