I experienced this too on my first run. It was the boss where you identify which color crystals will go off. Exactly as the poster says:

1. Your alliance wipes (or nearly wipes). Others are still alive.
2. The LB goes down to LB2 as if someone disconnected, but nobody is disconnected (nobody had the ??? over their name).
3. At some point, the LB then reset.

This effectively meant we could not use healer LB3. We still defeated the boss, but it was a whole lot more work for the healers.