I’m also having trouble with that node.
I’ve voted up UE-75799.
In my findings it is actually firing and working, BUT the failure node just seems to fire way too early. Despite a failure being returned, My Game Center login is successful, and after that point all other nodes I call work as expected. I may just have to build a delay into things as you mentioned.
I’ll try to remember to test on a fresh device to confirm that no login prompt is generated.