Greetings and thanks in advance for all help. Performing long overdue maintenance and repairs on my heating system. So far have found frozen boiler pressure relief valve and slow leak on the schrader valve on the pressure tank. Pressure tank was undersized (per manufacturer spec), so moved up to larger tank am now trying to balance the system. During the work, took the time to put a pressure gauge in the system between the autofill valve and the "system". fortuntely before me, someone put a shutoff about 2 feet from each side of the autofill so it could be isolated, the gauge I put in is between the autofill and the system side shutoff.
Thought it would be good to test the autofill to see if it was also in need of replacement, kept opening/closing shutoff and adjusting autofill till I got it set at 15psi on the new gauge. Then I left the system side shutoff OFF but the supply to the autofill ON to see if the autofill would hold pressure. Stayed like that for about 5 days and I'd get down there every day or so and it held pressure at 15 PSI no problem. This morning, I was about to finish the work and balance it and the gauge is at 40psi. On the water supply side of the autofill is what I think might be a pressure reducing valve? It is cylindrical and has a bypass tube to allow water to bypass it. I am assuming it reduces street pressure to a pressure that the autofill can handle?

Anyway, being that the autofill (standard watts 1156F autofill with lever) has a working range of 25PSI but a maximum pressure of 100 PSI, I am suspecting the watts autofil is flaky.

Is there any reason to suspect that the system pressure reducing valve has failed and is sending spikes to the Watts that could force it to pass the water through, which would then jack up the pressure at the gauge and shutoff that are a foot away?

Finally, is this something that just "occasionally happens", i.e pressure spikes from the domestic water system and being that the watts held pressure for days before going up, I should just forget it and move on?

FInally, in terms of piping, this run of shutoff > system pressure reduction valve > autofill > shutoff > system is "T-eed" off the main water supply about 15 from where it comes into the house. The boiler is the only applicance using water on this branch of the water system. There is one other "takeoff" from the main water which runs the length of the house to kitchen, laundry room, bath rooms.

THANK YOU!!