The mix album tag is set on MC as expected but it seems that either the ipod doesn't care or the tag info isn't getting across to it.
I think the problem mostly occurs when a largish number of tracks are being synced. For instance, where I find an album that is not sitting correctly within compilations on the ipod, if I remove it from the ipod, sync, then add it back to the ipod, the album will be correctly tagged on the ipod. If I were to do this with 100 tracks it wouldn't work, with 15 or 20 it works fine.