Your story chimes with that of my mother. She is almost 85 but not diagnosed until about 80. She had been a fit lady, exercising every day. In retrospect and having read up on this forum and elsewhere, the signs were there (frozen shoulder and loss of sense of taste and smell many years ago) but despite many trips to GP (and being told it was arthritis) there was a tardy diagnosis.
We knew it would be progressive and arranged for her to move in with us. It took a few months to move house,make adaptations (thought we were preparing for the future), etc. We were surprised at the speed of decline, including mobility. Then one day she rang me and said she couldn't move out of her chair. I lived about 90 miles away and drove straight down. Couldn't think what to do so just had to move her into unfinished (but almost finished) rooms. She did regain most of the mobility she still had before that but I have observed that there are sometimes step changes.
I think what you describe is quite typical - in the context of everyone being different, this seems to be in the area of something lots of people experience. For my mother it was part of the decline. Her situation has been complicated in that she has (shortly after moving in with me) been diagnosed as having Lewy Body and Vascular Dementia as well so it will be different for your dad. Hopefully he will regain ground. His medical practitioner might assist with alterations to medication. I hope you have good local medical support - it is a bit crap round here with no PD nurse/specialist.