"scan for new items" always scans the entire collection

Every time I open DK it scans the entire collection (~70K pictures, 14K 
videos) even if I select "Fast Scan" in Config->Miscellaneous->Behavior. 
This takes a very long time. If I uncheck "Scan for new items at 
startup", it does not scan anything at startup, but scans everything 
again even if I launch "Scan for new Items" from the Tools menu.

I'm running DK 9.1.0 with MySQL external (mariadb) but the thumb db is 
on a local ssd. This "always-scan-everything" behavior happens when I 
run DK on the "server" or a "client" (also with a local thumb db). I can 
let it finish scanning the entire collection (long time), close DK, open 
it again, and it will do the full scan again when I ask to "Scan for new 
items". During these scans "top" shows digikam, exiftool, and mariadb as 
the top CPU users. (In Config->Metadata->Behavior, the option "Use 
ExifTool backend to read metadata from files" is unchecked).

Any idea why this may be happening?

Thanks!