I think, we have a misunderstanding here... If you don't have commitment, then there is no carry-forward and your negative budget is caused by something else.
Now, if you reconstructed commitment and succeeded with carry-forward, then there should be no problem. Since I cannot look into your system, I'd suggest that you publish printscreen of FMIOI table for this PO. If, even after this, I'll be unable to see what the problem is, I'd suggest you to go and check with OSS service. To my opinion, there is no bug, just something that you miss/skip in the process.