Odd Quirk to my S9 AutoSet Setting

[ShelaghDB]

My S9 Autoset is doing something i can't so far seem to figure out.

I have fluctuated back and forth recently between a FF face mask and a Nasal version.

Without changing any settings for it, whenever I take OFF the Nasal Mask, within 2-3 seconds the machine turns off, which is how i like it.

But its not doing this for the FF mask. It just keeps going.
I have looked at it repeatedly and can't see which button would do this let alone differentiate between the masks.

Any idea what I must do to make it go off when I have the FF mask on?


TIA