Just curious, what pH probe are you using?
I will say it’s a TDS probe so you’re kinda on your own, but the good news is that the underlying principles are the same, which means that calibration would likely take care of any offsets. From your observations, I am almost certain that you have some other electrical connection in your measurement fluid causing galvanic effects with your TDS probe (such as a pH probe, or a capacitive level sensor, a flow gauge with metallic parts, or even a ground wire dipping into the liquid). Any other electrical probes inserted in the system WILL affect the measurement, if you haven’t done the capacitor hack modification I showed. All that being said, @Roberto has been using the same hacked probe with good results and reported them here.
Did you remove all electrical devices and probes from the liquid you are measuring? Not just for the calibration? It is important to identify the source of the galvanic offset!