We did the intro part with the infested bits and the cauldron, then entered and killed the Necramechs (T2).
Afterwards the mission showed the green exit markers until someone found the arcane vault and started waiting, after which all markers vanished.
I could set my own markers and see Ayatan stars etc, but no mission marker was present.
In conclusion, this is a very frustrating bug I've run into twice now, apologies to the poor randos that got shafted by this.