Tried thatClamp the body, pull the ring back and hold it while poking the o-ring back in position?
Also basically tried all of that ...^^That! I've had essentially the same thing happen a couple times with universal poppets is a corny keg just after fillingI used a metal pick to hold the core centered and a blunted orange-stick to work the o-ring back into place.
It is possible the o-ring has worn down a bit and it could happen again. Can you put up a picture of the other side of that PRV so maybe we can figure out how to disassemble it, maybe check for crud inside or replace with a buna or edpm o-ring?