I have had multiple conversations (and disagreements!) with my providers about my diagnoses . This is an important part of their care for me. I would not keep seeing a healthcare provider who wouldn't discuss my diagnosis.

It is not unreasonable to want to know a provider's diagnosis for you, and why they're using that diagnosis - that is, why they think it fits you and/or why they're recording that rather than perhaps some other diagnosis (e.g. "adjustment disorder" vs "major depression").

Also, I do go in to my primary care doctor (or any other specialist) and present my symptoms and then I may say "so I've done some reading and I wonder if this is related to X". It is important to be an informed patient.

Raising questions about diagnoses can help clarify where the issues are - and that can direct treatment. EMDR may not make sense if your T thinks you have major depression, but if you have cPTSD it may be something worth considering.