I think it depends ‘where’ you are when pressing the key combination.
If you are in grid mode, it clears the sequence (of trigs).
If you are not in grid mode, it seems to clear all sequences in the pattern and not the sounds (as you have pointed out).
From the manual:
“[FUNC] + [PLAY] (while in GRID RECORDING mode) clears active track.
[FUNC] + [PLAY] (while in no mode or LIVE RECORDING mode) clears pattern.”
In order to clear the sound from an individual track and return it to init, TRK + Clear seems to do the job.
There’s probably a combination that resets the whole pattern at once, but I’m yet to find it.