Armour transparency isn't applied to breastplates when Show Me Your Skin! is active. The mod applies a variable transparency to certain players or globally with certain paramaters such as if the player isn't in combat.

I could use the hide in armour setting but that would defeat the purpose of this mod.

@Bitslayn, this should be fixed for version 1.6.6 of Show Me Your Skin! for 1.19.4 and 1.20.