Can an employer force you into a negative PTO balance without your permission and during FMLA?

I work for a major health system. I recently had to take unexpected FMLA for a week prior to a scheduled vacation. I was off on FMLA for 1 week and returned to work for 1 days prior to my vacation. I had a week off for a vacation. I did not have enough PTO to cover both my FMLA and vacation. I was put into a negative PTO balance during FMLA even though company policy states you PtO bank will only go to zero. I requested my vacation unpaid which was not granted and now I am negative 40 hours. Is this legal? I work in Michigan. I know the FMLA part isn’t. When I taped to HR they could not provide me a policy. I am not being investigated by upper management and have a meeting in a couple of days with senior administration regarding my Use of PTO. I have always been an exceptional employee and never disciplined for my PTO usage. I have been asking a lot of questions to HR. Something isn’t adding up. Thank you.