I have discovered the same problem on my matrix with a slight difference, the base sound is always present on the D key and above- quite annoying
and if I move the octave button it changes position on the keyboard.
Example
Octave in 0 position of the note is D above middle C
Octave in -1. position of the note is D one octave higher.
I also have set the Matrix to Panel mode and disconnected all the filters to oscillators 1-3 using Noise only as a sound. The Base sound is still present and also not effected by the filters. This is a serious bug. I am hoping it is a simple fix but it doesn't make me very happy. If moderators are on board here I would love a reply. The Matrix is very new to me and I can still return it if this is an indicator of what I can expect. I have no reason to want a synth that has problems in my studio.