I’m embarrassed to admit that I was informed by support that I had set a breakpoint on an action within a workflow! 
I guess I should have known to check that, but honestly, I also think it points to another way in which the editing experience could be improved. Just like this suggestion regarding disabled workflows, if there was a breakpoint “badge” on any workflow/action containing a breakpoint, that would have made the issue immediately obvious.
PLEASE, Bubble, simple UX improvements like that this could go a long way towards a more pleasurable user experience, and would likely even save someone a bit of time.