The bug
All types of helmets and boots float in the player's hand.
[media]How to reproduce
Obtain any helmet or any boots in your hand
Enter third person view (by default, press F5 twice)
❌ The helmet or boots appear floating in your hand at certain angles
Expected results
The helmet or boots would not float in your hand, and would be touching the player's hand regardless of the viewing angle.
Attachments
Comments

Can confirm. Also affects 1.16.5.
Relates to MC-92274.
Can confirm in 21w15a.
Can confirm in 21w16a.
Can confirm in 21w17a.
Can confirm in 1.17.
Can confirm in 1.17.1.
I am able to confirm this behavior in 21w43a. Here are some extra details regarding this problem.
The Bug:
Helmets and boots float when held in the player's hand.
Steps to Reproduce:
Obtain and hold any helmet or any boots in your hand.
Switch into third person. (Hit F5 twice).
Look closely at the helmet/boots held in your hand.
Take note as to whether or not helmets and boots float when held in the player's hand.
Observed Behavior:
Helmets and boots float when held in the player's hand.
Expected Behavior:
Helmets and boots would not float when held in the player's hand.
Can confirm in 1.18.2.
Can confirm in 1.19.