In my experience with the sea to summit eVent bags they do slowly let air back in, but its a non concern because you can easily compress and pack before it expands.
I use the schnozzle too, but as a dry bag not a pack liner. I always cap the nozzle after compressing to slow the rate of air re-entering. You can also roll up the length leading to the nozzle for similar water resistance as the top opening.
I have tried the cloud packing method @rocco99 describes with trash compactor bags and find it very frustrating to compress my 20degree bag to make room for any other gear. Everyone describes it as so simple, “fold/roll over the top of the bag, compress/remove air, continue to pack” but I find that middle step nearly impossible with my insulation fighting my every move wanting to fill my whole pack. What compression I do achieve feels uneven, as if its only compressed at the top and puffed up wasting tons of space at the bottom (I guess it wouldn’t be wasted space if I could fit the rest of my gear on top but I can’t so it feels like a waste to leave the insulation uncompressed).
Now I am trying to compress my sleeping pad, quilt, warm camp clothes, and sleep clothes into one sized small schnozzel and loosely pack my shelter around it to fill the ‘wasted space.’ Compared to stacking the shelter on top of my dry insulation, packing around the dry bag can be shorter, leaving more room in the top of the pack.
The schnozzel dry bags are slightly tapered, so while you can get a nearly rectangular, oval, log shape, it does naturally want to taper, leaving plenty of ‘wasted space’ around the bottom.