Fixed by FirstPersonModel 2.1.1

The issue does not occour without FPM installed.
Looks like nothing I can do in Better Combat to fix this.

Not just Mythic Metals. Immersive Armor have some issue too (all avaliable armor sets) (Wooden Armor for example)

Phoenix Armor from Paradise Lost (aka Aether Reborn) and some armor sets from Twilight Forest (Knightmetal Armor for example)

Thanks for the report!

Dragon Armor (Helmet) from DragonLoot has same issue too