Strange, well I posted this in a different part of this forum
Edited Covers
All of the covers shown in that screenshot are hand edited, with the process of selecting the Cover in the FILE and therefore not BEST anymore for those specific albums. Since these are bookmarked I am 100.000 percent sure that these were manually edited. Since they all have the LP covers and not the CD covers.
after the update to 1.6 some had changed and had different album covers, yet with higher resolution - but not the covers I had selected. This never happened in 1.5.
It happened with many more files, but here for these specific albums I have a screenshot that I had posted here in december.
After changing all the wrongly changed covers back to how I wanted them, I have happily not found any more changes (but also I have about 4000 albums, so have not checked every single one)