Not a bug but a necessary feature. Tanks, I think all of them, can use several two types of ammo. Should they come loaded, you wouldn’t be able to unload the ammo to excahnge it for something else as you need free had to unload and the tanks come with a fixed weapon in both "hands". Now this wouldn’t be much of an issue in campaign short of wanting to load two tanks each with different kinds of grenades for their auxilary launcher, but it would make it impossible to change ammo in custom battles.