German Brewing

I've had this same problem (leaky keg/spunding valve). In my case, I eventually found I had a few slow leaks.

-On one of the points of the valve assembly itself (which I found by submerging it in water with internal CO2 pressure in the valve. Found the bubbling spot and fixed it).

-Also my gas post was allowing gas to escape with the extra weight of the valve, causing it to tip lopsided and lose the seal. To fix this, I wrapped a decent amount of plumbers tape around the gas post, with a little keg lube. Makes it a bit harder to get the QD on, but it's solid, and the unbalanced weight of the valve assembly is not compromising the seal. I suppose oversize rubber post seals would be similar in effect.

- Thirdly, my keg lid's pressure relief valve was not screwed down quite as tightly as it could be, and was also a slow leak.

When you are sure you have all your leaks sorted, test it by filling your empty keg with some CO2, say 10 PSI or so. Connect your Spunding valve, and take careful notice of the reading. Then come back and check it a few hours later. Keep checking it. Leave it overnight even. That gauge should go nowhere (unless there is a noticeable temperate shift in the room). Then you know you have it sorted.

Is your gas post shortened? Is the keg full to the top? The less headspace, the more responsive that small volume of gas will be to temperature changes. I've seen mine drop 4-5 psi as I cool to lagering temps.

I have not shortened my gas post yet so there is about 200-300 ml of head space. On the to do list.

It depends on the temperature. Eventually, as it gets colder, your reading on the PRV will drop. That's normal. If the temperature is constant and it drops, then you have a leak.