I have to agree with you about the personality and cognitive decline which occurs in Parkinson's disease when there is a dementia aspect.
My husband became dehydrated because of his concern over his incontinence and uninary frequency issues. He told me he was drinking water but in truth he was not.
The cognitive decline (a dehydration symptom) occured essentially overnight but took almost a week to diagnose.
The point is that during the ensuing weeks, he would ask me who the man or men were who were coming to the house - no, that was me calling the dogs in from the yard - and questions on that order. The implications are the same and things can get kind of ugly.
He's pretty much recovered now and taking medication for an enlarged prostate. (Yes, herbals had kept it under control for a few years but he had stopped taking them although he told me that he was.)
The other result is that I had to take control of his meds and the nutritional supplements and recently switched from a 28 day organizer (which can be used for 4 days with 7 slots per day sorted by meds and time) I now use a weekly (7 slot) pill organizer for one day's meds and supplements because he often switched rows and was taking overdoses by going down rather than across. Even though he is better, the fact that he tried to conceal the fact that he wasn't drinking fluids was enough to really scare me.
Eventually there will be cognitive losses for all PwPs with or without the dementia.
It is difficult at times to relate to this different person who is not the same person one knew for so many years. And that in itself is hard on all parties.