Unfortunatley FMLA only protects you for 12 weeks. Anything longer may or may not be unscheduled. The USPS can come after you for your attendance. It doesn't matter if you have cancer, a broken back, whatever. If you are over 12 weeks you are NOT PROTECTED BY LAW.

Just because you have a 3971 saying the PO knows you are absent does not protect you from discipline.

Arbitors in many cases have ruled in favor of employers because they must maintain some semblance of a business model. Obviously I don't know your specific case. You should seek counsel with a experienced steward or arbitration advocate.

I sympathize with you because we all the ugly actions from the USPS causes harships for our family. I hope you make a full recovery.