Has anyone had hospice discharge a patient for not declining fast enough?

Yup .
Dad improved and was sent to rehab . He improved a bit more and rehab was extended . Then he declined again until death a few months later.

Just need to say, Hospice does not determine discharge Medicare does. If Medicare feels the client no longer meets the criteria needed to continue care they will have Hospice discharge the client.

Sure. You get qualified for hospice by actually BEING at the end of life. Medicare accepts the opinion of your doctor that your demise is expected within a six month period.
Now, if at the end of that six months you are still in dire decline and death seems eminent, you can be RE-qualified for Hospice again. Rarely we hear about yet another requalification. But if your assessment proves that you are NOT dying, and are no long expected to die eminently, you may easily receive notice that you do not qualify for end of life care, because you are likely not at the end of your life?

I hope that makes sense?

Yes.

Yes, my mom. She was in and out of hospice for 2.5 years before finally passing at age 95.

Yes, of course. Hospice is to last for 6 months, and then the patient has to requalify again every 6 months. So if the patient is improving, or thought to have more than 6 months left to live, they are released from hospice care until such time as they again decline and are believed to have 6 months left to live.