so I did as you said at different row of holes and I got these 2 vectors plot.
What I can see is that even though the reverse flow doesn't actually affect my outflow of gas from the pipe, they form some weird pathway, for example at the bottom corner and the bottom part of the pipe.
So can I treat this reverse flow not as a problem, but more as a replication of reality? Although I still don't really understand how it happens. The faces of pressure outlet has a lower pressure than the fluid domian, so how can fluid still flows in?