Steps to reproduce: 

1. Using barriers, create 3 fully transparent 1 wide x 2 high cages side by side
2. Disable mob spawning, set the time to night, set difficulty to easy, give yourself night vision without particles
3. In one cage, spawn a zombie. In each of the other 2 cages, spawn a drowned
4. Using dispensers, equip the zombie and one of the drowned with a full set of armor of any type
5. Compare and contrast the heads

Observed results: 
The drowned's helmet is apparent only as some recoloured pixels.(compare with unarmored drowned) Whereas, the zombie's helmet covers its whole head.

Expected results: 
The drowned should wear the helmet in the same manner as the zombie

Notes:  
First noticed this in v1.16.40, when drowned were able to naturally pick up and equip items. This happens with baby drowned also.
At first I thought it was intentional, but then on very rare occasions I saw the helmets worn correctly.
The leggings also do not completely cover the drowned legs.