Garmin/Avidyne OBS weirdness

I fly an SR22G2 with the Avidyne MFD.  When I try to use the OBS mode on 
the Garmin 430 in VLOC mode it works fine -- the left knob on the MFD 
controls the OBS, it draws a nice purple line, and everything is hunky 
dory.  But when I try to use OBS in GPS mode the MFD doesn't work.  I 
can still set an OBS heading on the Garmin directly, but that's 
***bersome, and the purple heading line doesn't show up on the MFD.  Is 
this a bug?  A settings problem?  Something else?

Thanks,
rg