Able to replicate this 100% of the time using a dedicated server and one client.

Confirming issue. Until it is fixed, a workaround is to set the ammo of a vehicle to an amount that is just under 1 round of ammo(but not less than a half of a round).

For instance, for I_G_Offroad_01_armed_F
in the init or spawn information, put in the following:
thing setVehicleAmmo 0.002

When testing mission, the gunner will see 0 rounds, but it is in white. And if you then setVehicleAmmo to 1 later on, the ammo will be full.

I haven't tested this with vehicles with multiple ammo types.