I ran into this tonight, and I think I have figured out what triggers this bug.

1. The first time the Ebon is damaged enough to sink, the following round the goal changes to defeating one head of each color.

2. Once you finish the third of these goals, the Ebonimmediatelypops back up (instead of reappearing at the end of the round, like the rest of the respawning heads).

3. If, once it resurfaces, you damage the Ebon through Epics or AOEs in the same round as it first reappeared, then it sinks again without resetting the goal back to killing one each of the other three heads.

4. The next round there is no Ebon head and there are no new fight goals; the battle is now stuck with no way to advance it.

From my experience this is how it works; there are four types of head: -

Cobalt, Scarlet, Amathyst and Ebon.

Ebon is the one you need to kill to win but it only appears at certain times.

Round 1, kill 1 Cobalt, 1 Scarlet and 1 Amathyst; then the Ebon will appear. Fire everything you've got at it, it will disappear after about 1/3 health is gone.

Round 2, kill 2 Cobalt, 2 Scarlet and 2 Amathyst (these get easier), then the Ebon will appear. After about 2/3 health has gone it will disappear.

Round 3, kill 4 Cobalt, 4 Scarlet and 4 Amathyst (these are easier again), then the Ebon will appear a final time. Kill it.

If you do not kill it this time I believe you have failed and should flee.