This happens when you go to change the unit's settings (The Name, the damage, the health)
The Siege Tank (Siege and Tank mode) is bugged and it places it always 0 damage.
You need to change their turret damage.
Wrong forum
I do change the turret damage, once I open up that window again... it's back to 0
ADDITION:
Plus it's a bug, I can't make it to scmdraft forum or starforge forum when the title has both names
Doesn't do that for me... Please explain with great detail your exact steps.
Moved to general staredit.