Official comment
I recall another user having this issue back in the day, but unfortunately I can't recall what the solution was, nor can I find it in a search.
It could be that something was spilled and the edge of the button is stuck to the housing (a small amount of rubbing alcohol on a cloth might help clean it), as those two buttons may be on the same board and pressing hard on one may result in the other contact switch being activated.
You could always send it for servicing at Pioneer or find a local shop to clean and repair it for you.