Minecraft version 1.19.2
Forge version 43.3.7
ColdSweat veresion 2.3-b02e

Armor slot won't accept armor. Insulation slot takes armor.

I'm pretty sure this is working as intended. Leather chestplate is classified as an insulation item, so it goes in the insulation slot, and furthermore, insulation items can't themselves be insulated. Hopefully this clears up the confusion.

Ah ok. I think I was recalling how it worked in a prior version and thought it was a bug.