Mobs suffocating on boss arena transition

When the boss arena transitioned from Default to Wither, a piglin suffocated and the gold nugget phased upwards to the surface. I couldn't see the piglin so I'm fairly sure it was underground before the arena transitioned. It appears that there may be a "cavity" below the boss arena where mobs can spawn, and when the arena transitions, those mobs suffocate? (I don't think piglins are meant to spawn, but this could also be an issue for overworld hostile mobs?) Minor issue but silly