FX FX Dreamline PowerPup breech leak

Recently acquired a new to me FX Dreamline PowerPup. Using the .22 barrel (stx superior with both pellet and slug ports) I noticed a big puff of air coming from the breech. Replaced the breech seal and one outer ring (closer to breech) but did not seem to make a difference. Using either pellet or slug side did not matter

Also tried the .177 barrel and that worked splendidly, no air puff in the face

Any thoughts on why the .22 is pushing air out the breech but not the .177? Could it be leaking through the notch on the top of the transfer port meant for ?

Thanks!