I’ve not been playing much of late but thought I would drop in for the last few hours of the Simulator in Slice 5 - which has 2h and 35mins left from the time of this post.
Did opening mission and it wouldn’t let me go into the “Round 3”. I’m like… ok, maybe a bug. Sometimes a restart helps. Closed. Restarted and now gone. Closed again. Restarted still nothing.
If you were locked out of the sub event after completing the initial fight while there was still time left, I would open a support ticket. (Include your screenshots!)
They obviously won’t be able to let you play, but if there was a bug that prevented some people from joining Simulator after Galactus Hungers started, they may extend compensation. It probably won’t be much, but hey, free stuff for filling out a web form!