Playing on Steam/PC, solo, Cattie-Brie, Act III of the first level. At the boss fight with Garnn and Murdunn, I defeated all except Murdunn. He disappeared -- I think it happened directly after all other enemies were defeated but can't be sure.

His status bar was there in the HUD, but no boss, no arrow indicator that he was hiding somewhere. I wandered around the play area for about 5 minutes trying to trigger him, but he never showed up. So I quit the level (***) and restarted.

On the second try, I did not encounter any glitches and was able to defeat him.