I received the following response from Tech Support - Windows

“Hello Tonnie

If you are using slash chords then the program will play the closest variation possible since we did not record the actual chord Cmaj7#5/Ab. That would most likely be a C/Ab chord. There is a Cmaj7#5 for that style but not as a slash chord.”

Now I will try the following solution:

1) Freeze the bass track, to avoid a boring bass line of only C's
2) remove the slash chords, so the piano will play the Cmaj7#5.

Brgds,
Tonnie