I've been experiencing the same issue for a couple of months now. I recently investigated the problem a little further, here's some info I posted on BIS forums earlier this month:

When connecting to an AL-6 carrying leaflets (whether I use pylon settings or the "addmagazine"/"addweapon" commands in the drone's init), in the UAV Terminal , I see "Leaflet Dispenser" and "1 Leaflet" listed in the drone's weapons (WPN). However, when taking control of the drone, the weapon info window in the upper right corner remains blank (no weapon name or remaining mags/ammo), and pressing the fire button or attempting to switch weapons does nothing.

Scripted leaflet drops (e.g. through the use of waypoints with the "fire" command) appear to work just fine, though.

I have done further experiments to isolate the problem, and it seems to affect all single-seat unmanned vehicles in combination with the "Bomb_Leaflets" weapon. Single-seat manned vehicles (such as aircraft) also seem to experience the same issue, although I can't be sure that it shares the same root cause.

However, unmanned vehicles with separate pilot and turret seats seem to be able to operate leaflet drops manually to some extent, but only if the "selectWeapon" command is used in addition to the commands given in the leaflets instructions link above.

For testing purposes, I attempted the following:

Adding the "Bomb_Leaflets" weapon and "1Rnd_Leaflets_West_F" magazine to the AR-2 Darter drone: I was able to do a manual leaflet drop, after adding the "selectWeapon" command in the init first. ✔️
Adding the "LMG_RCWS" weapon and "200Rnd_65x39_Belt" magazine to the AL-6 Pelican: I was able to use/fire the weapon while controlling the Pelican drone. The weapon and ammo were also properly listed in the weapon info window. ✔️
Adding the "Bomb_Leaflets" weapon and "1Rnd_Leaflets_West_F" magazine to the UGV Stomper unmanned vehicle - unarmed version (only other unmanned unit without a turret): not working - same conditions as the Pelican, although the weapon info window doesn't even appear.❌
Adding the "Bomb_Leaflets" weapon and "1Rnd_Leaflets_West_F" magazine to the A-164 Wipeout single-seat aircraft : not working.❌
Adding the "Bomb_Leaflets" weapon and "1Rnd_Leaflets_West_F" magazine to the BTR-K Kamysh APC : The weapon was assigned to the driver in the weapon info window, but I was unable to fire. Switching back and forth from the driver's seat to the gunner's seat removed the weapon, which had been assigned with the "selectWeapon" command. Attempts to assign the weapon to the gunner's seat using the addWeaponTurret/selectWeaponTurret commands were unsuccessful. ➖
Adding the "Bomb_Leaflets" weapon and "1Rnd_Leaflets_West_F" to the AL-6 Pelican using the addWeaponGlobal/addMagazineGlobal commands: not working. ❌
Adding the "selectWeapon" command to the AL-6 Pelican init after addMagazine/addWeapon to force it to use the leaflet launcher: not working. ❌

It should be fixed in devbranch.