So you are saying that a leaky O-Ring/lean carb will cause the pipe to push away from the flange (likely when revving), therefore bringing air back into the cylinder when the pipe bounces back?
If the pipe is pushing away from the cylinder when you rev the engine then you have a problem with the springs that hold the pipe in place.
For a two stroke pipe to work correctly it has to have some movement, not locked completely down tight. Thats why its so hard to maintain a seal anywhere the pipe has a connecter joint.
If your pipe is pushing away from the cylinder, yes you could at times suck raw air back into the pipe just from it doing that.
A two stroke engines tuned pipe is a very complicated item in how it functions & the dutys it performs.
With each rotation of the crank & cycle of the piston there are a bunch of things happening inside a pipe.
You have a pressure wave then a suction wave then a pushing pressure wave that pushes anything that was sucked out back up into the cylinder.
All this action inside the pipe is happening with in the time the piston movement uncovers the exhaust port opening untill the time it covers it back up.
Now, think about all that action happening from 0 to 10,000 rpms