The bug
When summoning an endermen holding a head or skull, or a block display entity with a head or skull block state, the head or skull won't render.
How to reproduce
/summon minecraft:enderman ~ ~ ~ {NoAI:1,carriedBlockState:{Name:"player_head"}}
/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:player_head"}}
In 1.16.1