YANBU.
My sister and I shared joint Power of Attorney both medical and financial of our mother until she died this spring. I would have told sis everything relevant when mum lived near me in accommodation and when she moved to nursing home near sis, sis would tell me.
I think your brother is bring unnecessaryily secretive, and I do think he is coming across quite weird. Only you know why there is guilt and what that may be doing to his judgements. I don'Y think we need to know that.
As your 'care' of your dm is shared you are perfectly at liberty to ask this professions what the visit was about. Especially if you have shared PoA.
In your shoes I might simply approach the professional with my ID and copy of PoA. If he asks you why you would need to know (why would he?); you can admit your mother has no memory of the visit and you need some notes of what was discussed.
If your dm has dementia as mine did (she had Lewy Body but your dm may have a different kind) them a one to one with a health professional is pointless without a n adult who knows your bring there. As you know/found out, your dm may not remember a thing, and may not be able to give correct answers. Even for a professional to truly access your mum would (IMHO) require another person present who can answer questions like "How long has she been like this?' Or 'Is this normal for her now etc?
So a few thoughts occurs to me:
Your brother was present at the meeting and has not admitted this
Your brother is lying about the meeting
Your brother is lying about even knowing this person )do you know he knows her/him)?
In my own mum's case she was on XYZ medication, she often felt anxious and wanted to try something different or feared a different medication. When medications changed it was never good and after years we just stuck with what seemed best. It would be tempting to think a, b, c could change her condition but my sis and I knew not. It would have been very hard if my sis and I had not seen eye to eye.
In your shoes I' would pursue the professional because you have joint PoA . Then I would get my db round for a meeting about mum, or choose a neutral location, and get all the cards on the table.
Joint PoA requires you to act in mum's best interests and so to me it is implied if you do not agree you still need to work together. it is encapsulated in the word JOINT. Your Dh needs to share his concerns and you need to listen, you both need to work together.
Sorry to say this but, when she is gone, how you both handled this time together will probably have a huge impact on how you deal with her passing.
Good luck. 