Occurs with multiple resource packs.
Screenshots taken with Default resource pack

What I expected to happen
When on partial armor is placed on, the other empty armor slots render like normal with the armor outline

What happened instead
When the chest plate was equipped without other armor being equipped, the pants and boots squares would instead render as flat dark grey, as would the pants if they boots were then equipped.
If hovering mouse over the chest plate this then corrects the rendering until the mouse is moved. If hovering over the boots square while glitched, it goes a slightly lighter grey, if hovering over the pants square, it reverts to normal until the mouse is moved.

How to recreate
Just equip armor
I haven't been able to get it to happen in many worlds. I made several worlds trying to recreate this and only manage to recreate it in one other, so a rare glitch as far as I can tell.

Quitting and reentering my world, restarting minecraft and restarting my computer all have no effect. Going into other worlds and returning to the affected worlds also has no effect