ok. I think I've figured out why (or rather) what scenario triggers this behaviour.
anytime there are songs from more than 1 album in a playlist, somehow the same song info is used for all the songs who share the same position (or index) in their respective album.
for example, in a Michael Jackson playlist containing songs from Thriller and Bad, when the phone says it's playing Billie Jean, the car will say Billie Jean. but when the phone plays "another part of me", the car will say Billie Jean because in their respective album, both songs are at the number 6 position. otherwise when all songs are from the same album, song info is in sync across phone and car.
I cant be sure that's a true correlation but that's what I've observed with limited insight into either device.
am not sure why it sometimes says unknown album and track, I wonder if it's because there's no track index info.
a good test for someone with a different phone or earlier version of Windows phone is to check if there's track number display on the idrive when streaming from those phones.
let me know if anyone's observed something that's contrary to my findings.