When the keyboard scanning is set to "Full Range" it is making rather sophisticated determination of what chord is being played by both hands and then passing that information to the accompaniment system.
If I were to play the notes C, E, G (C-Major Chord) with my right hand but also play a F note in my left hand our products will identify that you want C chord over an F. This is notated in music by writing C/F. So the accompaniment bass line will be centered around F, while the other parts play a C chord. As you noticed, it does this with many combinations that make musical sense. This is sometimes called a "Slash Chord". Sometimes is is used when just one of the notes from the original chord is played in the left. Example if I play E in my left hand and C, E, G in my right this is an Inversion and would be noted as C/E.
What is this feature called? It is called Music.
A note in the left hand that isn't part of the top chord is often a passing note
as it moves to a different chord. Countless examples of this but here is one that I love. Watch and Listen at 2:39
in this video - https://youtu.be/Q8kWq3kfGrg?t=158
Notice the D/F# - while just a chord inversion it has a (dominant) sound that begs for the next chord to be a G. \
So there is nothing wrong with the keyboard, it is supposed to identify chords this way. It identifies what is happening in the baseline vs the rest of the parts.