On lower difficulties, debt is capped at -2000 (don't recall if that's still the case now, but it's no bug).

Its hit the cap for that level of difficulty - so it shuts down lots of resource to claw back cash until the economy is back in balance - "balance" in this case is -2000bc.  It probable that a close inspection of income bearing assets will show many if not all at zero, providing the means to rebalance the debt.  Such drastic action also affects the stated income figure depending on how its chosen to forcefully rebalance everything and whats left, that has not been shut down, to show 'income'.  It will also shut down a production facility thats operating at a loss  (especially those) - further complicating the visible calculation.

Regards
Zy

I think the answer has been given already. There's a maximum amount of debt, based on difficulty, that has nothing to do with the -500bc.

That number on his difficulty is -2000 BC. In other words, if you spend 75000BC upgrading a fleet of large ships, and click TURN, on that difficulty level, your debt will be reduced to -2000 automatically, whether or not your empire is turning a weekly profit or defecit.

Okey dokey.

In both screenshots Income is larger than Expenses so he has positive income.  Also the Treasury number is in orange indicating it is below zero but going up.

I believe Kyro has given the official Stardock response.  Debt is capped at -2000.  If you get below -2000 by say upgrading too many ships, you will be automatically brought back to -2000 next turn.  A helpful 'cheat' at the easier levels I guess...