1. Create a mission with a German grenadier (with a Kar98k) and a blitz truck (or a HEMTT passenger truck. The bug does not seem to be related to the specific vehicle.).
1. Play as the grenadier
1. Switch to rifle grenades (or don't)
1. Mount into the rear of the truck as Passenger (Right seat) or Passenger (Left seat)
1. Fire a rifle grenade if you have one loaded
=> Succeeds
1. Try and load a new rifle grenade into the rifle
=> It plays the reloading sound, but does not load a round into the barrel (or play a reloading animation)
=> The round disappears from your inventory
=> The gun can not be fired.
(You can keep reloading new rounds, but they just disappear until you're out of rifle grenades)
1. Dismount
=> There is no rifle grenade in the barrel.