My bank has a negative cash balance but the game does not force me to do anything about it. Save game attached.
This has happened a few times before but I could never consistently replicate it.
Basically if you want to make it appear:
1) Turn off capital ratio notification.
2) Stop all deposits on a new bank.
3) Wait until you run out of cash.
There is also another problem I noticed at the same time: If you select Stop All Deposits, this does not stop HQ Deposits, only public deposits: