Can't kill shield generators with bombs alone

I was running Practice Mode with a B-Wing, and spawning in a star destroyer, and making bombing runs with proton bombs. I was flying under its shields and mosquito-ing up to the shield generators, and dropping all my proton bombs at once. I discovered something interesting.

If you drop all your bombs onto the star destroyer's Shield Generators, using only proton bombs (no laser file), they will go down to 0 points (as shown in the targeting computer with it focused on that subsystem) but they will not explode. No matter how many repeated bombs you drop, the generators stay alive at 0 points, and the shields of the star destroyer stay up full.

If you add a single blaster shot to the generators, then they blow up real good.

This sounds like an unintended bug to me.