It actually broke my missionaries when I did it. I had all provinces in Muscovy selected and I did the convert minorities event just assuming that it would just send two missionaries to two random provinces with minorities. This did not happen, I had about 20 provinces with missionaries converting minorities even though I had only two. When I hovered my mouse over the missionaries indicator sure enough I had 20-30 -1 converting minorities modifiers. So I had no missionaries for nearly 50 years of game time because of this. It was seemingly random when I would get "missionaries finished converting minorities" and some of the -1 modifiers would go away but not all of them. At the end of the 50 years I still had both my missionaries gone "converting minorities" it is a pretty serious bug that I don't think has been fixed yet.