This is just a general observation since you provide zero information in respect to what framework or cart script you are working with.

In most event driven workflows the event manager, dispatcher or the event object itself will expose a mechanism for allowing you to stop the propagation of an event based on some criteria. I'm curious to whether if you stopped the propagation of that event if your action would have been performed and it solved your issue.

Often times in this type of situation the most productive approach is to just breakpoint the workflow until you find your offending code.