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.
![Smiley :)](https://forum.sequential.com/Smileys/default/smiley.gif)
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 !