I'm not sure, this has happened to 4 seperate provinces in the heartland of my empire, hence they have not been attacked yet since bugging. I can however spend a buttload of gold and buy them back to +125 again.
EDIT:
No I do not get gold back increasing a negative PD. And gold is not an issue when you poke memory address 01055EE0
(which of course has no bearing on this bug)
And, several turns later all 3 of my TC immortal heroes were slaughtered by that evil Hero Munching Horror, which for me unfortunately is a game breaker, so it's back to Marvel Ultimate Alliance until that is fixed.
I guess that is the definition of Karma.