Yes it usually is from peak of roof towards the pipe (right to left) .Looking at your first pic from outdoors. Your pipe is on the left side of the roof peak.
What direction does the prevailing/normal wind blow from?
Right to left by any chance?
Check the wind direction the next time you have back puffing,
Post what you find.
Keep working at it. Just a puzzle that needs the pieces put together correctly. Don't give up!!
I'm definitely gonna keep at it till I figure it out !
Thanks
Ernie