I had the same issue - it was due to me changing part of the conditional values from text to an Option Set - but not updating the text values after I had made the change. Although the expression looked correct, and it didn’t highlight in red suggesting an error, the expression wasn’t actually resulting in a correct value.
When I clicked the end part of the conditional, I could re-select the Option Set value instead of the text value - so it was a really quick fix once I had sussed it out.