Summary:
Endermens sometimes place a block inside itself.
They cannot place blocks inside other entities, but they can sometimes place blocks inside itself.

Steps to reproduce:

1. Summon a lot of endermen with a block in hand.

   /summon enderman ~ ~ ~ {carriedBlockState:{Name:"minecraft:dirt"}} 

2. Watch them.

Observed results:
Endermens sometimes place a block inside itself.

Expected results:
Endermen will not place blocks inside itself.

Video:
enderman_placing_pumpkin_inside_itself.mp4

Screenshots: