There is no animation for second hand items (My character is right handed, so in this case, his left hand items) when I switch to a loaded crossbow. The right hand has a smooth "falling" animation of what ever item it is holding when I switch to a loaded crossbow, but the left hand item doesn't. It just flicks out of existence (But just to clarify, this is just a visual bug, I still have the item). I noticed this when I had my shield in one hand and one of my tools in hte other when switching to the crossbow. I think that the left hand's animation should do the "falling animation" like the right hand does, but if this turns out to not be a bug, I guess I am suggesting the addition.
Can confirm in 20w51a.