When it wants to…
Which is almost never
Are the shells normally present on a “fully loaded T-72”?
yes
I think it’s an issue with module damage. Because WT is a game and everything has to have some sort of HP pool, there is problem with distribution of damage.
Projectiles themself often deal too little damage but it’s the combination of the projectile and the damage from “spalling” (armor fragments) that results in expected behaviour.
The game doesn’t really care that some projectile penetrated an ammunition module, it only checks for the damage inflicted to the module, which results in some random chance for detonation based on the inflicted damage.
If you deal enough damge you can detonate ammo probably nearly 100% of the time, but when when a projectile only make ammo yellow or orange, it’s just RNG whether it has any effect or not.
A lot of time when something is not working as expected, the developers will make some changes that artificially cause that behaviour, which often results in some other things causing problems and no longer working as expected.
For example overpressure. You can have the your entire crew get killed by pressure that would kill the crew in some area of the tank, if at all, but in-game it knocks out everyone inside the vehicle.
It’s an oversimplified mechanic that can result in very stupid deaths compared to the damage of other ammunition types.
I guess ammo pool existence is also decided by RNG. I had a moment that I have 15 rounds in my Leopard 2A4, which carrying only inside the back of the turret. Then I got hit from the hull and my empty ammo pool right next to driver is blown up, and the kill cam showed me there was a single piece of ammo, which there wasn’t. (Got hit by T-80).
Sounds like a bug to me.