Basically, if the message that didn't autoscroll properly has a
timestamp above it then you're experiencing the problem I quote in
the link above. I plan to release the definitive fix for that
particular problem by next Friday (in the mean-time I explain how
people can fix the issue in current versions of Propane).
I tried to reproduce the problem you described (using 1.0.8 with
the fixes explained in the link above) and was unable to.
That said, if the message that didn't autoscroll properly
doesn't have a timestamp above it then there's clearly
another problem that I've not seen before and my fixes are unlikely
to address your particular issue.
In that case, I'd appreciate a screenshot of the Propane window
after it fails to autoscroll so that I can try to recreate the
messages prior to the failure.
I'd also be interested to know if the autoscrolling 'resets'
after you speak some text (2 messages in a row, temporally far away
from the 5-minute timestamp cutover ) into the room too.