Second surgery denied even with mod 79

HI. I received a issue from a fellow coworker. She has a patient who had a foot amputation done which has a 90 day global period. Paitent had a sig flex done during the global period. That claim was denied by Medicare Part B as part of the global period. A mod 79 was attached. When she call Medicare to state the mod should be valid, they said it is not and we should be billing with another modifier.

Has anyone else seen this? The sig was not part of staged procedure. Any advise or...

Second surgery denied even with mod 79