The Rampart blueprint protects the Bombard from self-damage
Here is a Bombard with Overdrive active and a Rampart blueprint shooting an enemy door. Next to is another Bombard with Overdrive, but no Rampart at all. They are all inside the Garrison radius, if that makes any difference. The Bombard with a Rampart blueprint attached doesn't seem to suffer self-damage, while the other does.
Game Version:
Steam Public
Platform:
Windows
I can confirm that, somewhat.
The Bombard on Overdrive who is hitting something will love HP, the Bombard which doesn't hit something (just the imp. Wall) will not lose HP.
Those Bombards shoot without hitting the door.
I double checked your report, and Bombards on overdrive with a Rampart Blueprint with some defences, does also lose HP.
Btw, thanks for the picture, it helped me replicating your issue easily and very quickly.