Just ask yourself: If you go diving, what would have to go wrong for you to benefit from a BC that gives you a guaranteed face-up position?
You would need to be:
1. At the surface
2. Unconscious
3. Breathing
4. Wearing an inflated BC
5. With no DV in your mouth
6. Without a buddy
The only way this could arise is:
You loose consciousness underwater.
Your buddy initiates a controlled buoyant lift, but lets go of you - deliberately or accidentally.
You go up to the surface, your BC inflates due to decreasing ambient pressure, your reg falls out of your mouth.
Fortunately, you arrive at the surface without bursting a lung or becoming so badly Bent you die.
Your boat doesn’t see you, or is unable to send rescuers.
Your buddy does not surface very soon after.
It’s fiction. It’s unadulterated nonsense. You have a better chance of winning the lottery without buying a ticket than you do of winding up in need of a BC that guarantees you a face-up posture. You are (genuinely!) more likely to be injured by a toilet than by a BC that floats you face down.
The only BC in the world that guarantees you a face-up position is an Adjustable Buoyancy Life Jacket (ABLJ) - the old horse-collar style BC. Stab jackets do NOT give you any such certainty, so if you decry a wing because it doesn’t float you face-up, then you’re either using an ABLJ, or you’re a hypocrite, or you’re ignorant. Sorry.
My wing is, incidently, set up so that it pulls me onto my back - it's more comfortable that way. I have to make an effort to go face down.