@mbi , that is frustrating isn't it?
This was my experience, maybe it is true for you:

I was sent to cardiac rehab six weeks after my open heart surgery.
It was approved by the hospital coordinator and insurance for six weeks.
On my fourth week the nurse told me there was an issue with insurance and I was not approved afterall.
What? Why?
I had a choice, I could pay for the rest out of pocket ($248 per visit, or something like that) or not come anymore.
Because I felt fine I chose not to go anymore.
Then several months later a bigger confusing insurance issue arose, and even though I was told there would be no charge, it was the hospital's mistake...I was sent to collections.
Grrr!
That made me mad.
Long story, longer...I was told because of the diagnosis used, insurance would not pay.
It's a congenital condition in their opinion because it's genetic.
Totally messed up response in my opinion, because open heart surgery is the reason for the rehab. It should not matter why... your chest was cut open!
I was not on Medicare at the time, Blue Cross of Texas.
Never-the-less it was never paid. And I fought the collection agency and won.
Good thing I kept good records about all the misinformation!
Anyway, that's my little rehab fiasco story. It had to do with the ICD-10 codes used and they ultimately did not approve it.