Finished off the fight with Lagon, he just says he tires of this and gives you a bunch of loot. Stepping onto the moon on the floor does not take you anywhere. Lagon doesn’t talk to you either, just tells you to leave. No portal options. Teleporting out just kills the run and you have to do both bosses all over again 
Done this twice, made sure to pick up all loot / kill all adds from 2nd phase. No luck. Softlocked woo!