say turn 1 they have 5000 income, 3000 expenditure then they get 2000 bonus; assume their reserve is 4000, right?
then they spend all of their money to build and recruit, thus at turn 2, they have 5000 income, 3000+4000 expenditure, 2000 bonus and X AMOUNT of increased upkeep.... based on these calculation, their reserve would be gradually smaller and smaller each turns
so in the end, at certain point, they would have no more reserve

and since I assume AI money bonus is not in an increased size based on upkeep, how would you explain this issue?