This may be intended behavior for hostile mobs in Peaceful difficulty, but this causes all Ocean Monuments in a world to lack Elder Guardians upon switching to Peaceful, as Elder Guardians do not respawn.

Steps to Reproduce
1. Spawn or summon an Elder Guardian
2. If on Easy difficulty or harder, change to Peaceful
--> Elder Guardian does not spawn on Peaceful, or despawns upon switching to Peaceful.

Expected Results
This may have negative implications on unexplored Ocean Monuments as Elder Guardians may no longer be present, as they cannot respawn upon switching out of Peaceful. Elder Guardians should not despawn in Peaceful difficulty but rather follow the behavior of Vindicators and Evokers in Woodland Mansions - turning passive in Peaceful.