The parameter Mono/Poly determines how many notes simultaneously can be played by a selected instrument. In the real world (and in synthesizers) some instruments are designed, on purpose, to support just a single note of polyphony(sic). Meaning they play just one note at a time or what is referred to as MONO.

Flutes, oboes, trumpets, clarinets, trombones, bassoons, tubas, etc., etc., all your pipe based instrument in the real world support just one note at a time. (A bowed and/or plucked string also exhibit this same behavior - they can only support a single tone). This causes a unique "behavior" as they attempt to transition to a new note on the same breath of air. This transition is referred to as "legato"... You will notice on several of the pipe based instruments you will either see "legato" in the name, or perhaps the word "solo" or even the designation "AF1" or "AF2" attempting to direct your attention to the fact that they may provide this mono-legato behavior under certain conditions.

Call up the Voice: "Flute Legato", or "Soprano Legato" you can play only one note at a time, just like on a real flute, or real soprano sax but importantly, each new note does not repeat the same attack. You can connect notes in a more organic realistic way. This is because in order to sample (record) each note, an actual player must blow each note in the scale so each can be mapped to a key. Naturally they attack each note individually. So when you strike the key you playback that original recoding -with the original Attack.

This means it's like taking an "in" breath before each note. "Legato" is an articulation that lets the note trigger while you are still holding the first note... It will transition to an Element edited so it starts without the musician's Attack. The XA CONTROL (Expanded Articulation) detects your playing style, if you trigger a note while one is still being held (on these "legato" Voices) it selects the Element without the Attack portion. This gives you a smoother transition and allows you to create more musically natural phrases.

Synthesizers like the MiniMoog (circa 1971) were capable of playing just one note at a time, because polyphony was really, really expensive back then; the MiniMoog sold originally for $1495 and made one glorious note at a time. It naturally exhibited this MONO behavior. It's pretty hard to believe you were playing three notes at a time and not realizing you are only hearing one of them...

We mentioned this mono mode earlier in this thread, forgive us for not picking up on the fact that you didn't know what was happening. Yes, some of the Voices in your Motif XF are purposefully set to this MONO mode. This means they only sound one note at a time.

"Sweeper" and "Opening" are two such Voices. Play a chord, listen closely - only one of the notes you are playing is sounding... At a time. The most recently struck replaces the earliest struck.

Now please, with this knowledge re-read this thread. I believe you will discover that you were cancelling out the one and only note your selected lead sound was playing with your left hand chord - exactly as I suggested earlier.

I am very grateful to you Bad Mister.
thank you so much for your excellent reply.
I will check this out carefully and report back.
you are an excellent teacher.

just re-read the whole thread.
and played my XF.
you nailed it Bad Mister.
thank you again for patiently explaining this all to me.
very very excellent.
thank you.