Approved treatment and then denied treatment

My insurance provider just did something to me that no one has ever done.

Back in August, they approved a course of treatment (monjuvi & revlimid).

This month, they say the treatment of monjuvi is not covered.

I feel that the treatment was working as I was walking better.

Multiple attempts had failed (R-CHOP & Radiation; Polatuzumab, Bendamustine & Rituximab; CAR-T)

Thanks in advance,
Michael