Didn't really know how to explain it in the bug reporting tool. The goblins in the goblin grand room falls through the floors if they're not agroed. Seems there is a few seconds of a "grace period" before they kinda fall..
If you fight the goblins where they stand, they might still fall through the floor. I've been able to consistently make them not fall by agroing them and getting them to come to me. However it sometimes looks like they're stepping up from slightly below the floor when they charge, and that forcing them to move makes them step onto the actual floor and stick around.