Since you have presumably eliminated the contact strips as a culprit, and you say there is a pattern to it, this can be either a problem with the voice circuitry, or with the keybed (i.e. either the key contact circuit board or the wiring to it). And the diagnosis below works for most keyboards - not just the Matrix 6.
If certain keys do not play - for instance if F4 (and other keys) never sound - then this indicates a keybed problem. I don’t recall the specifics of the Matrix 6, but a broken trace on the contact board or a broken wire will make a pattern of notes across the keybed that won’t play, like every 8th note or every 12th note as you play chromatically will not work, and those same keys will always not work.
Since the Matrix 6 is a 6-voice synth, if every sixth note does not sound, no matter what keys you play, then that indicates a problem with the circuitry on one of the voices. So if F4 doesn’t play one time, but then it plays fine a few notes later and now D5 doesn’t play, this indicates a problem with the voice circuitry.