Have a look at why the flow is reversing before changing anything else. Is there an explainable cause? What is the DPM condition set at the outlet? What is the outlet backflow condition, and how does that compare with the flow going out near there?
Simulation is a tool, and you need to understand why something is happening rather than (always) worrying about the warnings. We use the warnings to focus our checks, but don't always correct them.