Similar to 1.15+, any action using the interaction key (right mouse, such as equipping armor) or dropping items would swing the player's hand. Small QoL visual change I would like to see!

I will leave this open as it's a valid feature request, but I don't plan to work on it in the near future. Modded items are able to control the swinging themselves so I would only be able to fix this for items that use vanilla code.