A stand alone PC.
That simplifies things then. The problem could only really be that the "TheTVDB Series ID" tag is wrong for the Series that have the incorrect Cover Art, as CountryBumkin suggests.
I know you said you deleted all values in that field though, as below, and MC worked for a while, then the bad Cover Art came back. That can happen in a Client/Server installation, but not in a Stand alone PC installation. At least I can't think of how it would.
I ended up deleting all series ID, then the Cover Art > Get From Internet seemed to work for a while, but soon after if reverted back to the default, generic cover art.
I assume that you are aware that "TheTVDB Series ID" tag is relational, which means that there can be only one value for each Series. So if you change the value on
any Episode belonging to a Series,
all Episodes get updated with the new "TheTVDB Series ID" tag. So if you are doing maintenance and change the Series name of some program so that it now matches another series name, and you change the "TheTVDB Series ID" tag on the second file, all files belonging t the new Series name will be updated.
Could something like that be happening?