Some info
This issue is an inconsistency between the three types of portal blocks. It is a re-evaluation of MC-54462, as since then, end portals and end gateways have been changed to be unobtainable.
The bug
While nether portals are rendered while being held by endermen or as block display entities, end portals and end gateways are not. One could expect that end portals and end gateways are rendered as well while being held by an enderman or as block display entities.
How to reproduce
Endermen
Run the command:
/summon minecraft:enderman ~ ~ ~ {NoAI:1,carriedBlockState:{Name:"nether_portal"}}
Run the command:
/summon minecraft:enderman ~ ~ ~ {NoAI:1,carriedBlockState:{Name:"end_portal"}}
Run the command:
/summon minecraft:enderman ~ ~ ~ {NoAI:1,carriedBlockState:{Name:"end_gateway"}}
Block displays
Run the command:
/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:nether_portal"}}
Run the command:
/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:end_portal"}}
Run the command:
/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:end_gateway"}}
Observed result
End portals and gateways are not rendered while held by endermen or as block display entities, though nether portals are.
Expected result
All of the mentioned blocks would be rendered while held by endermen or as block display entities.
Can confirm for 1.16.2 pre-release 2