You may find it valuable to look up a similar question asked by Jasir Jawad on Feb.22, 2023 here on ResearchGate:
How to fix reverse flow on the pressure outlet in fluent?
I am simulating a cylinder filled with water vapor which has one outlet (no inlet). It is a pressure outlet. I am expecting to see flow through the boundary as a result of pressure difference between the body itself and the outlet boundary condition. However, as soon as I start simulation, reverse flow occurs and eventually leading to crash. My outlet boundary has lower pressure than the cylinder initial pressure. The simulation is transient and with compressible flow.
Several answers have been offered there, including a discussion of the natural phenomena.