I just tried it with USMC_Soldier and USMC_SoldierS.
In both cases i had the same problem.
The interesting thing in all tests was:
If you open the map, click on units, select yourself, open your invetory an click on rearm you are able to select the grenades again.
(i habe a localized version so i dont know if rearm ist the right translation for the buttons caption ;) )