I don't think my CPAP (S9) has any auto on/off function so I really don't know anything about that, but I wonder if you can just take the machine back to where you got it, as safely as possible, and ask them to service or exchange the machine.
I know it won't fix the hissing air hose issues but I wonder if there is a way to avoid bloating whilst continuing to use the CPAP.
Personally I find that my machine delivers effective results at anywhere between 10 and 13 with very little change in comfort or events. The deciding factor for me is that by dropping from 13 to 12 I can pretty much eliminate bloating. Obviously this process is usually achieved through official channels but you should be able to have some input if you understand what you are aiming for.
Another consideration that I have not seen addressed as yet is that different masks might deliver different outcomes when it comes to bloating.
Logic would seem to imply that having larger outlet vents and a corresponding increase in airflow may make a difference so perhaps those masks that everybody complains about being too noisy could be worth a try.
Another option would be a larger hose or some kind of inline reservoir like what the plumbers use to prevent water hammer. I don't think any of those options exist as yet so maybe they don't work.
One option that might exist is a longer hose.
Any of those options (in theory) could act as a pressure moderator and limit the peak pressure created by expelling air that forces it's way into the digestive tract.
Of course Sierra will inform me that they would all interfere with the normal function of the machine. :(
Have a look at this Clinical Manual, and in particular page 14 in how to access the Clinical Menu, and the top of Page 17 were it talks about SmartStart. SmartStart is what controls the automatic on and off. Your machine should have the ability to turn it off...
Also keep in mind that the machine goes into a cool down mode when you turn it off in the morning. The puts a small amount of air flow through the hose. However it is much less than the big rush of air that you should get when you just remove the mask with the machine running.
I noticed this as well on my A10 machine at the same time I switched from a nostril mask to a nose cushion mask. I changed the mask and the setting on the machine.When I switched back the problem didn't happen anymore, the machine turns off as it should. I usually turn the machine off manually rather than have it switch off by itself, it might still be happening but I don't notice.
I wondered if it had a software update, I wish it gave a notice for updates, but I am a technical person so I like seeing that information.
If you go go into the configuration menu under the Clinical Menu at the very bottom there is an an "About" section. In it there is a SW number for the Device, Humidifier, and Modem. Not sure if the SW stands for Software version or not. In any case my Device SW is SX567-0306.