While developing the flac plugins, I came across this as well -- there is a post somewhere in the developer forum, but I'm obviously too lazy to go find it
The problem is (or at least was) that MC uses several different fields and is not consistent. There is "Year" as well as "Date". Date is a number (an integer, actually, representing a number of days) while Year is a string. IIRC, when reading tagged info from the files it looks for "Year" first followed by "Date", but it writes only "Date". "Year" is apparantly deprecated and will not be carried forward. Not sure if it's in 11.1 or not.
In any case, the decoder plugin is what determines how the info is used and stored in the files. However, the info will generally be maintained in the database unless the database is updated from the tag info -- this is where the day/month/year info is replaced by only "Year". I haven't tested it, but the conversion probably causes and auotmatic update from tags since it would need to get new file size, bitrate, etc. info from the new file.
You might try converting to a new file and then copying the tags from the old one to the new one afterwords. Then delete the old one. That way, you will at least be able to maintain the tags.