Start from scratch.
Are you just turning off the filling valve after topping up to 1 bar or are you also disconnecting the filling (flexible) line?. If just shutting the filling valve(s) water could still leak in, build up pressure leading to the PRV lifting. I would definitely disconnect the filling line to rule that out, for a start.
If no problems with above then problem definitely points to the EV (expansion vessel), it could have a punctured diaphragm resulting effectively in having no EV as it would be filled with water, it could also have a pre charge pressure far too high, maybe 2bar, if the pre charge pressure has fallen to 0 bar then the pressure would only have risen to ~ 1.7bar with 5 rads on but would have lifted the PRV at 3.0 bar with all rads on.
Guesswork is no good, get the EV checked out ASAP, I would then also get the PRV renewed as it might still dribble now that it has lifted numerous times, not worth taking the chance.