Sounds like the 'instant hot' hot water backflowed into water heater causing the PRV to leak.
A check valve on line to instant hot should solve that.
Course I don't know how you have it plumbed.....
It could have just been because you lost the 'air gap' in top of water heater tank.
And when water heater made hot water IT caused the PRV to leak.
To re-establish air gap
Turn off water supply
Open closet facet Hot side to relieve pressure
Open PRV via lever.... (CAUTION WATER COULD BE SCALDING) when water stops flowing out let it snap closed
Turn on water supply
Close faucet
ON EDIT:
This is the same problem you posted about early.
You also posted you have three valves on water heater......3 valve system don't use check valves.
You need a check valve to stop backflow from instant hot
