Your doctor is trying to protect you from the inherent risks of high dose and long term prednisone use.

One of the side effects of prednisone is called euphoria. It is the feeling of euphoria and reduced anxiety that prompts people to use too much of this dangerous drug.

Another definition of euphoria is a "false sense of well being." If you have PMR, you are not well no matter what prednisone is telling you.

People should use an abundance of caution when using prednisone. Try not to take more than your doctor prescribes. If you do take more than prescribed, as I did ... make sure your doctor is aware of what you are doing. Someone will need to evaluate you because you won't be able to trust what your body is telling you. In this case ... don't listen to your body.

One of my very first experiences with high dose prednisone was when I thought my eye was a "lost cause." I was certain I would be blind in one eye. Then I proceeded to tell my doctor that "I didn't care if I ended up being blind." My ophthalmologist said I was feeling "too good" and wanted me to reduce my prednisone dose immediately.

You need to learn how much prednisone you can tolerate. You won't know what dose that is when you first start to take it. I needed to learn my limits in spite of what prednisone was telling me.

This all stems from what cortisol does to the body. Read about the "fight-or-flight" response. It is a cascade of things that happen when there is a true emergency. Prednisone stimulates this fight-or-flight response but PMR is not a true emergency. Granted PMR is painful and that pain relief that prednisone provides gives you that false sense of well being.
https://www.britannica.com/science/fight-or-flight-response
The fight-or-flight response is not sustainable in the long term.