Instance crashes during boss fights - losing map

This is ridiculous - I've lost 2 Nexus maps, 1 Expedition boss in Logbook and several tower bosses this way.
The instance is crashing during the boss fight, being rolled back to the hideout after disconnect, resulting in disappearence of all the portals (despite having revives left).
The loss to progression is massive at this point.
Please address it asap, otherwise the endgame is unplayable.

If it is connected to any skill crashing the instance, the only one I can think of is Spearfield.