It seems that since Anomaly 1.5.3 update there's an issue with grenade launcher rounds:
Reloading a save or moving between maps while having the UBGL active on a gun transfers the amount of ammo the gun had loaded in the main magazine to the UBGL (UBGL can thus have 30 rounds after reloading a save).
Shooting and pressing V twice lets you shoot again until you run out of ammo.
Detaching the UBGL or unloading the gun returns all grenades to your inventory.