Ran into this twice in a row. I defeat one of my allies, interact with the rift and the ally doesn’t join me in the fight and Mordremoth is permanently invulnerable.
A second bug we managed to work around earlier in the same fight was that I couldn’t interact with the reality rift (as a sylvari) when Mordremoth was trying to control my character. Please look into this Anet, I am not going to keep getting that far into the fight and not be able to complete it.
Vee/Volk
Maguuma – Predatory Instinct [HUNT]
Necromancer