Changing album artwork corrupts mp3 files

I have a collections of mp3 files some with crappy old artworks. When I have the time, I upgrade some of those images to nicer ones.

After I upgraded to Mojave, I sadly realized, that the files I change become corrupted.

The sympthoms:

After the artwork is changed, the end of the song gets repeated. It means when the song comes to its end (e.g. fades out), iTunes still shows like 20-40 seconds remaining and those last 20-40 seconds of the song will be repeated. On the Mac iTunes detects the real end of the song and skips the repeated ending, but Quicklook or iTunes on iOS does not.

I downloaded the new images from the web as I just did before, all different sources and resolutions (500x500 or larger), but it was never a problem before. Now the issue happens consistently for all files I edit (about 300, updated separately or in batches) after the Mojave upgrade. I have a backup so no data is lost but it's like I can't edit files now.

Does anyone else have this problem and found a solution already?

(BTW I change the artwork the normal way: iTunes -> select song -> Get info -> Artwork -> click to select image and delete with backspace -> Add artwork -> browse to JPG file -> OK.)