While it would seem like you have a problem, in that logically I would expect an internal pressure to drop when what is essentially a pressure vessel is vented to the outside, all that tells us is somewhere you have probably made a mistake, in some way. The issue is, you don't show any code, so the only advice anyone can give you at this point is to find and fix the mistake. (Duh, that seems a bit obvious.) What it will probably entail is to look more carefully at any and all subsystems, and their interconnections. Is everything doing what it should do? Break large problems down into small ones.
If you want better help, then you need to post the code you have. Then show what you see as a result. Explain why you think it is predicting nonsense. Then someone else can test your code and verify it does indeed fail, and also they can hopefully show how to fix it.
