I had recently the following phenomenon:
Diablo clone spawned (due to the sold SOJs on PTR) instead of one of the sigil-bosses (Grand Vizier of Chaos)…after killing the diablo clone, breaking all the other sigils and defeating there corresponding bosses, the regular diablo did not spawn!
I checked all sigils two times, they where activated and nothing happened…
Looks to me, that the diablo clone doesn’t count as a slain sigil boss and therefore the real diablo can’t spawn!
Not a very dramatic bug but a little bit annoying i suppose 