If it doesn't go back to

default setting[/b] (i.e. 12 panel tones, with no split/layer/rhythm and tone selection is on "Modern Piano" mode), you can try this:
1) Adjust all the needed individual settings back to default after you've played through your registration memory banks...and by going back to default I mean...
> remove the split-mode
> turn off the rhythm
> remove the layer
> press the "Modern Grand" button
>

THEN SAVE TO any one of the 96 registration memory banks[/b] (remember where you store it as this will then be your "Back-to-default-mode" registration memory bank)
> every after you finish using your registration memory banks you would always go back to this specific registration memory wherein you stored the "default mode settings"
2) Alternatively, upon turning on (i.e. booting up) the PX-320, while its still on default mode...
> try storing, say, in Bank 1 Area 1 (i.e. 1-1) this default setup so that Bank 1-1 would then become your "fail-safe button" to go back quickly to the default settings/Tone-mode of the PX-320 every after using a registration memory setup.
The aforementioned are just suggestions or "work-arounds"...its worth a try.
Also, I'd suggest e-mailing Casio Tech Support to report this issue so that if there is indeed a better way to shift back to Tone mode from registration memory setup and it wasn't mentioned at all in the manual, it can be addressed accordingly.
Hope this helps.