EDIT : I´m home now testing, and it seems to work the way you described that it should do.
( As described in the Manual )
Pick a program / sound without unison : plays normal.
Press unison and
fat unison sound is activated.

Press unison again and plays normal.
Hold chord, press unison and display says "CHO", release unison and any one key plays chord.
Press one key and press unison : goes back to normal.
However, when pressing unison again without holding a chord it goes to CHO mode ...
So unison stays in CHO mode even when pressing one key .....
To get back to normal I have to press the same program / sound button again witch I started with .....
In your case there seems to be other problems ....
As you have a desktop P-10 I guess you have external keyboard or sequencer.
MIDI settings OK ?
Plan B = Contact support : /
Cheers !