The Immured Fury goes out of playable area on Forge maps.

Both times I have encountered The Immured Fury bonus boss on a Forge map, at some point in the fight he will slide off the playable area of the map, and down into the decorative 'lava' area.

This leads to having to move around in specific ways to bait him to come back to the traversable path.

Very annoying, likely a simple oversight.

Cheers!