It doesn't sound like a problem with the machine to me. I know I have problems exhaling on and off depending on body position, congestion, and I think the phase of the moon! Usually when on my back there's no problem. On my side I sometimes have problems.
For me the exhalation problem feels like nasal passages are open but I can't exhale unless I force it. This turns out to be a pretty common problem. A-flex/C-flex is one way to cope ... you could try increasing it to 2 or 3.
I use nasal pillows. When this happens to me I'll find myself exhaling occasionally through my mouth. Then it slowly builds up until I have to do so again. Since I can't sleep like that I put about 5 to 10 minutes in to see if the problem clears (sometimes it does) and if not I'm back on my back (that's sometimes a different problem because I have a bad back). I guess with a full face mask that really isn't an option for you. Unfortunately I have no real experience with full-face masks so I don't know if the anti-asphyxiation flap is causing the problem ... hopefully someone else will have more info on that.
If the increase flex doesn't help and it isn't the mask, you might become a candidate for Bi-Pap.
If I'd been smart I would have used some form of "Kevin" for my account name. Or BirdMan if I was going for anonymous.
I wasn't smart! But if you read this far at least you know my name now.