question

This isn't a bug, and using a Breathe isn't a work-around; it is the solution.

https://docs.flexsim.com/en/20.2/ModelLogic/AdditionalConcepts/TroubleshootingProcessFlows/#delays

Wait for Event fires immediately. It synchronously interrupts the event to execute additional logic.

The first setState() hasn't even finished when you are trying to set the state again. Of course you need a breathe. Let the original event finish before changing the state again.

The OnStateChange event fires before the state value is even set. If you return -1 from OnStateChange, then it doesn't set the state value. You are calling the setState() function recursively from within the setState() function before it has even set the state value from the first function call.

So the second setState() is finishing before the first setState() finishes, which leaves the object in the state of the first setState() because you interrupted it before it finished. Let it finish with a breathe if you want the functions to happen in series instead of in recursion.

Here is a diagram showing how Wait For Event works and what using a Breathe Delay activity does. (This example is in the setstate function, but the same principle applies to listening to any event.)