Please check from the Action Window > File Properties > File Type Info in MC11.1 that the ratings really are in the physical file tags before importing the files to MC12.
If so, then it may be an MC12 bug that must be dealt.
In general, you could restore an MC11.1 library backup file in MC12 for getting all old data available easily (including view schemes, playlists, statistics, custom library fields and all field data that is not available in the file tags).
Naturally, it is important that people test the new import system too.