I think that you missed one point about Event Manager configuration:
When you create the mapping for this type of event, the Target is the linking object between the two process objects. This contains two Reference Numbers – one for the process that the action is being performed on, and one for the process that is being attached or detached. You need to map to only one of these Reference Numbers – to identify the process that you are attaching or detaching
So I believe this has to do with mapping, but this is only suggestion.