I can say that in my case i had negative balance and it tuned back to 0 in a few days. So yeah, you can have negative balance (i think only when you used crypto as collateral) but then ftx turnes it into 0. So
no, i had to move fund there to zero it , i dont know what would it be after several days but i m good guy and moved them cause negative balance looks bad and i fell like i owe :/ if mechanism really zeroing negatives by itself i lost money unfair