Somehow, mainhand items switch with offhand items and vice versa.

Could you please elaborate on this? This could just be guards randomly being left handed.

I created a datapack that restricts specific items to the main hand and requires bread and a shield for the off hand. However, for some reason, about 1 in 10 spawned guard villagers have this configuration reversed, with bread in the main hand and the specific main-hand items in the off hand.

I didn’t realize that guards could be left-handed! Is that a feature? It would make sense if they have some randomness in their item placement.

All mobs can be left handed

Will close for now unless it's not just a thing with guards being left handed