I had that happen to me once. I reset it and it fixed itself. Kind of a pain hopefully it is fixed for good.

even worse, it robs you of all prestige points the round is meant to give you.

Yeah. Just happened to me. negative in the billions

Just a thought: for overflow to happen, the actual accumulation-value should be above the maximum that a variable can hold: so simply inverting the negative values should only cause a reduced income, which is already a common result of offlining.

I guess the issue is that after a prestige your points are lowered by a huge amount and that amount is taken into account for offline calculation. It's only happening after prestige. I was even able to exploit that fact and do a prestige to get a high number of negative offline income to get a lot of negative points after switching back to the game. After that I prestige and since I go from a lot of negative points to zero, my offline income is calculated to be a very high number. I switch to another window, go back after a few minutes, get a lot of offline points, and can immediately prestige again.