1. Place down unit, set as playable in a new mission
2. Make sure unit has 1 grenade of any type
3. Play mission
4. Remove 1 grenade via _container addMagazineCargo [_grenade, -1] (e.g. (vestContainer player) addMagazineCargo ["HandGrenade", -1])
5. Observe that counter for grenade type is at 0x (grenade doesn't have to be selected when the code above is executed)
6. You can throw the '0th' grenade

addMagazineAmmoCargo is broken in same way.

removeItem, removeItemFromX and removeMagazines aren't broken.
removeMagazine is unknown, as it's broken but in a different way (see https://feedback.bistudio.com/T79813).

Interestingly, removeItems doesn't work on grenades, it throws the following error:

11:40:54 Warning Message: No entry 'bin\config.bin/CfgWeapons.HandGrenade'
11:40:54 Warning Message: No entry '.scope'
11:40:54 Warning Message: '/' is not a value
11:40:54 Warning Message: Error: creating weapon HandGrenade with scope=private

with more lines after that (but imo not relevant here).