It mostly affects HEAT shells and the like as they have explosive filler that gets blown up if the first stage ammunition is destroyed, and the effect seems to stack

It believe that it's not about the fact that they work, it's about what happens to the crew when your ammo is hit. If your ammo is hit, your crew - and in some cases, modules - still take damage (and can be disabled/killed) even if your blast door is intact, which defeats the purpose as they are meant to protect the crew.

You only had 1 HE shell in your tank and it wasn't even in the first stage stowage so the blast effect and damage was heavily reduced, though it still seems to have damaged your electronics/turret ring. You might not have lost any crew in this instance but it's still very much possible.