Off-hand slot sometimes doesn't play item pickup animation

The Bug

When switching between the off-hand slot and a slot in the hotbar with the F-key - or just putting items into the offhand slot (via F-key or via /item...replace) - the pickup animation only plays on the hotbar and only when the item stack that was moved there has more items than the other one.

Alternate way to reproduce:

/clear
/item entity @p weapon.offhand replace stone
/item entity @p weapon.mainhand replace grass

Observe how the item pickup animation only played on the grass, but not on the stone.