Beastmaster's Bane, confused

So the seasonal task of defeating Astaroth before the Amalgam is resurrected in Torment IV, it is not getting ticked off for me.

I’m literally killing Astaroth himself within seconds when he’s on his own feet while the amalgam is down in the pool, what am I doing wrong here?

His life bar goes to zero and Amalgam’s lfie bar goes up 1/3… and done it multiple times by now.

Welcome to the Diablo 4 beta test, season 9. The final version of the game with all bugs fixed is expected around season 20. Until then, you may experience various bugs.