I'm using mp3tag 2.52 and Audio Shell (www.softpointer.com) to view the encoder id. I have music stored in Flac format, which I currently convert to Lame 3.99. I then use mp3tag actions to update filenames, titles, comments, track, artist, discnumber, album & year, after which the encoder is changed from Lame 3.99 to FhG. The problem occurs even if I make my mp3tag changes to the Flac files first, then convert them to Lame 3.99 (most tags are duplicated in the Goldwave conversion), and then update a single tag, such as Artist or Discnumber.
I'm assuming only the encoder tag is changing, not the actual encoding of the file. This hasn't happened with all, or even most files, but when it does, it uniformly affects all files in an album. One album, containing 919 kbps flac files, does not have this problem, another, containing 1142 kbps flac files, does. Any mp3tag update, even changing a single character, results in the error, regardless of the Lame 3.99 bit rate. Adding an intermediate step of using Goldwave to do a flac to flac conversion did not change the results. Both sets of flac files use LibFlac 1.2.120070917, as does Goldwave.
While experimenting with files of different bit rates and Lame versions, I also found a file that was Lame 3.96 which had its encoder tag changed to Xing. However, my experiments seem to indicate it's mainly a problem with Lame 3.99 encoded files.
I first discovered this was happening with mp3tag v2.44.
EDIT: I tried to make test files, but I could not get the failing files under the 250k upload limit. I did notice that the file that does not produce the error also does not change size when updating with mp3tag. The test files that change to FhG encoder values after updating add 290 kb to the file sizes.