When you have a riptide trident in one hand and food in the other (and you are not in the water) your trident hand raises as if charging the trident, but the trident isn't flipped forward like normal, instead its just pointing backwards. does not affect first person view. My guess is right clicking triggers the player's model for using the item, but because the player isn't actually using the item, the trident or shield doesn't match the player's model.
Related to MC-178572.
Would love to see this fixed, as it's very misleading with shields.