Is every medical diagnosis objective knowledge?

No.

Only medical diagnoses which it is possible to prove on a physical basis are objective knowledge (e.g. myocardial infarction, pneumonia, .... ).

Symptom-based medical diagnoses (e.g. fibromyalgia, tension-headache, .... ) and diagnoses based on phenomena as it is the case with a psychiatric diagnosis (diagnosis of a mental disorder) can not be proven on a physical basis.

Immanuel Kant delivers the philosophy to explain this in detail.