arpeggiator drops first note in arrangement loops

[Clearscreen]

if you set an arp running into an impulse it illustrates this well.
set a one bar loop in arrangement.
then have a midi track going to an impulse with an unlooped (or looped, it doesn't seem to make a difference...) session clip triggering the arp to hold on 1/8 beat.
every time the arrangement loops the first note is dropped.

also, if you trigger the arrangement to play from a different start point (for example before the loop zone) the arp timing gets really screwed up and seems to give up for a while before it'll resync.

anybody else noticed this? i couldn't find anything with a quick search although this thread:
http://www.ableton.com/forum/viewtopic.php?t=50281
may be related?