I think the pharmacist is trying to "protect" you.
However, just as you have mentioned, usually a doctor can prescribe for "off label" purposes.

I never give my pharmacy my diagnoses. I've never heard of pharmacies requesting diagnoses.

Your doctor can call the pharmacist and discuss this. That's how my pharmacist and doc work out misunderstandings, etc. They get on the phone with one another and sort it out.

I hope this gets resolved soon!

WC