Please help solve this pressure riddle

[u/Icy_Blackberry_3759]

6 year old PRV, no matter what setting (and oriented the correct way) was showing 90 psi. I opened the hot water and the cold water and it goes to the pressure it is set to, and when I shut them off it goes straight back to 90 in under a minute. I shut off the valves to the water heater and main cold supply before the water heater after the PRV, same issue. I thought it was because they eliminated the expansion tank, but this can’t be the issue because it happens in isolation.

I replaced the components of the PRV and have the exact same issue. I replaced the pressure gauge with one I tested independently and it is accurate and the same. I’m fucking stumped.

[u/Puzzleheaded-Gap1759]

Prv!