No Coast Guard Type III PFD (the kind most of us wear) or Type V (inflatable) will float you face up when you're wearing a trapeze harness.

ISAF Rule 43.1(b) says (in part):"A hiking or trapeze harness shall have positive buoyancy and shall not weigh more than 2 kilograms, except that class rules may specify a higher weight up to 4 kilograms."

So where is most of this "positive buoyancy"? On your butt.

Try floating relaxed in a swimming pool dressed in full gear. The stable floating position is face-down, butt up.