Minions stopped respawning in boss fight

I was fighting the Fungus Behemoth in a corrupted Nexus. The map modifiers made it a bit of a long battle, and all of a sudden my minions stopped respawning. I made sure I was on the correct weapon etc. I ran around, tried switching weapons a few times and nothing would get them to respawn.

I was unable to beat the boss and failed the Nexus as a result. Frustrating since you have one shot at them but hopefully this will help others going forward.