I have seen this bug happen more than once, enough to know it's not a one-off thing. During the Strange Terrain strike, you are required to kill some yellow/orange health bar knights, which drop some orbs (called relics) that you have to throw at some crystals to advance in the mission. The bug happens when supers are used against these knights. The relics don't drop and simply disappear. I have seen Strikers and Sunbreakers, maybe also Arcstriders, cause this bug to happen. All this basically results in the fireteam having to wipe so that the event can be done again, hopefully without anyone using their supers against these knights and causing the bug to happen again. Wishing for a fix to this soon!