Noticed this small bug.
How to reproduce:
have 2 frag grenades (and maybe other grenade types, I think I had at least 2 smokes additionally) in inventory
throw a frag grenade after selecting it from quick slot --> the quick slot still does display that you have 2 frag grenades after that (Screenshot 1)
left click the quick slot --> nothing happens (grenade is not selected)
right click the quick quick slot --> now the quick slot correctly displays that you