So it's quite possible the Square D pressure switch is bad / not working correctly. I had a similar problem. I replaced an old rusted out Square D pressure switch that was leaking out of the bottom with a new Square D FSG2J21CP 30-50 PSI Pumptrol Water Pressure Switch. The new pressure switch seemed to be working, but every so often I'd notice the Stenner Pump Chlorinator to be running non-stop. First I made sure my Gould's Pump pressure tank was at 28 PSI (2 PSI less than 30 PSI), which it was NOT, so I lowered the pressure to 28 PSI. Make sure you use a good digital air pressure gauge (like the Topeak SmartGauge D2), and double check the air pressure with a regular air pressure gauge, to make sure they are both reading the same PSI. Unfortunately, the aire pressure was not the problem, causing the Stenner Pump to run for over an hour.
I next determined (by turning on a faucet) that my Stenner Pump would turn on when the water pressure fell to around 42 PSI, but the Pressure Switch wouldn't Cut on my Well Pump until the water pressure dropped to around 38 PSI. The pressure switch would Cut OFF both at around 48 PSI, so the pressure switch was not dropping 20 PSI (50 to 30) before Cutting ON and automatically turning on the Stenner Pump at the same time. I tried adjusting the Pressure Switch adjustment screw and the smaller differential screw, but it didn't change things much. Basically, I came to the conclusion, that in certain instances where water use would cause the water pressure to drop to around 42 PSI and then water usage was then stopped (so the pressure would not drop any further to Cut on the Well Pump at 38 PSI to raise pressure), my Stenner Pump would run non-stop chlorinating the water.
So I purchased a "second" brand new Square d pressure switch (exact same model, but this time from Amazon, instead of Lowes). I installed it using the same wiring as before, and the pressure switch Cut On at 30 PSI when the water pressure dropped to 30, turning on both my Well Pump and my Stenner Pump at the exact same time. It then cut off both at 50, like it should do, taking only 30 seconds or less to bring the water pressure up from 30 PSI to 50 PSI.
So if your having issues with a stenner pump running non-stop like I did, you may want to try replacing the Pressure Switch with a new one, and hopefully that will solve the problem.