Yay, one of the oldest issues on my list is now fixed. 
https://github.com/discourse/discourse/commit/4a1755b78092d198680c2fe8f402f236f476e132
There were also lots of other improvements that should prevent the creation of staged users when emails are rejected in the last couple of weeks. Please create a new topic if you find any other weird cases where staged users are still created for rejected emails.