Oh never mind, I think I got my answer on the v4 character sheet lol.
"Ability bonus is always 10 less than the defence.
The bonus can be negative."
Yes this is an implied rule that's never actually spelled out anywhere so I put it on the new character sheets to avoid confusion. Ability bonuses are always ten less than their defence, so when the ability defence is depleted by wounds or attacks, the bonus reduces by the same amount. An ability defence of 09 or less will give you a negative modifier until it heals, and can lock you out of succeeding at some saves if it gets low enough.