RTD reads differently with different monitors
They're really low-end non-isolated transmitters (not so bad for RTD but that's a really bad thing for serious industrial thermocouple applications).
Check that there is no circuit to ground with a multimeter. If there is none (I suspect that's not the problem) it may be responding to EMI from a VFD driving the AC motor. It's not hard for an amateur to design a circuit that rectifies the interference and produces a DC offset. Nonlinearity in the front end circuitry.
You can try to add capacitors across the inputs, shielding the sensor wires, shielding and separating the motor wires from sensor wires, etc. but I believe you'd be better off with a better designed product if you actually care about the reading. 50°C is a seriously bad offset (represents about 20mV at the front end for PT100 which is nothing short of horrible, it would be of the order of 500°C on a K thermocouple).