A core armor feature is not functioning yet.
Minecraft has begun trialing netherite horse armor. Not everyone trusted the developers’ claims and some players opted to verify the armor’s “fire resistance” themselves.
A quick lava test by user _cetera_ revealed that the supposedly fireproof gear still catches fire.
For some reason, various netherite items could be submerged in lava unharmed, while the new horse armor was destroyed immediately.
It seems the Minecraft team simply missed enabling the setting that activates the armor’s fire resistance. Mojang has acknowledged the bug and is preparing a fix.
Source: iXBT.games