If the expansion tank is properly sized and the pressure is adjusted correctly, AND if there's no additional water being inadvertently being introduced to the system (autofill valve leaking, a leak in the heat exchanger with the WH, etc.), AND, you've gotten all of the air out of the pipes, the pressure should be stable, hot or cold. The only way to correctly measure the pre-charge in the expansion tank is when there is no water pressure applied, otherwise, it will read the same as the water pressure.
Assuming you have an autofill valve, it may no longer be shutting off properly. Once the system is filled, you could shut off the autofill entirely as the system should be closed and never lose any water. If it does, the autofill valve will mask the leak problem and keep the pressure in the safe to operate range (unless it is leaking, then the pressure will rise).
12psi is probably the minimum pressure otherwise the safety pressure switch would prevent the boiler from firing. A common residential boiler relief valve is often 30psi, so the usual desire is to operate somewhere above the minimum and less than max...a figure of 14-16psi is fairly typical. You don't want it to be bumping off the minimum because it might fail to fire up. Too high, and it would be dumping water. Adding fresh water to a typical boiler is not a good thing. It works best when it is in there for the duration as it reaches an equilibrium with the metal parts.