In Minecraft version 1.17, a bug emerged where performing a right-click action that slows the player down using an item in the main hand causes the offhand arm to consistently face straight down, regardless of whether there's an item in it. Similarly, if the action is performed using an item in the offhand, the main hand arm faces straight down, irrespective of any item held in it. This issue affects various items such as eyes of enders (because of MC-107185), shields, spyglasses, as well as all food and drinkable items. This behavior was not present in version 1.16.
This duplicates MC-225931 based off other issues which have been resolved as dupes of it.