As title said , i upgraded from diamond to netherite and the insulation disapeared , didn't really notice before i went in the nether

EDIT: It does the same when i put an enchanted book on my armor , and only performance mod i have is embeddium if that helps

I couldn't replicate this issue when using enchanted books in an anvil, but it does happen in the smithing table. I'll have this fixed for the next version.