Known fact 1: An Enderman that picks up a blocks cannot despawn until he places the block back to the ground.

Known fact 2: An Enderman cannot place blocks on top of Bedrock.

Bug: If an Enderman in the Nether picks up a Warped Nylium or Crimson Nylium and they teleport themselves on top of the Nether (above Bedrock), then they can never despawn and always count for the mob cap. (Or until the player gets there and kills them, or if they teleport back down again to the Nether and place the block to the ground).

It happened in my Single Player server that, not 1, but 2 Enderman being at the top of the Nether both holding a Warped Nylium . I have no idea how long have they been up there lowering the efficiency of my Nether mob farms until I went up and kill them myself.