I think probably it is added back to the investors at the time of adding back. So if someone divested, he won't get anything, but if someone invested, he would get a share of the added back amount 
Yeah, that's how it sounds like. Actually when I designed the moneypot investment system, what I did was create a repayable log of all the investment/divestment/bet events for in a nightmare situation like this (or software bug) it could be replayed so investors wouldn't have made/lost money from the changes in the bankroll when a fake better (or software bug) was playing.
The situation is probably a big mess now, as some investors have lost more than they should've and others made more than they should've. And it's probably pretty likely the ones who unfairly made money have already withdrawn (?) or at the very least, will be unhappy if their balance gets put to the correct amount