During the MediaMonkey 3.1 development cycle, some changes were made to MediaMonkey’s Ratings values to fall more in line with WMP/Windows/etc. As a result, this feature in your program has been thrown off. Some ratings in MM appear lower in your program, while other MM ratings aren’t shown under the MM Rating field, but rather as a normal POPM entry.
Seeing as 3.1 has been officially released and this change is now out in the wild, you might want to look into adjusting things within MP3Tag, if you plan to continue to support this feature. It doesn’t contain information about what the values were changed to, but the relevant MediaMonkey bug entry for this change is here, I believe.