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.