You might want to read the instructions as I believe the sensor only works properly with distilled water or Double Protect Ultra coolant.

The High Flow Next spec says the conductivity sensor can measure from 2 to 200 uS/cm. The DP Ultra MSDS sheet says it contains 20 to 35% Ethylene Glycol and <1% Benzotriazole. Assuming the rest is distilled water, the conductivity of this mixture would be less than 1uS/cm. In reality, due to impurities, its typically about 10 uS/cm. The Mayhems X1 coolant MSDS sheet says it contains 40% Glycerol and 59% pure water. Assuming the same amount of impurities, this coolant should have a conductivity that is about the same as DP Ultra. There are some coolants (EK Cryofuel) that have extremely high conductivity that is far beyond the upper limit of the sensor in the High Flow Next, but based on the MSDS sheet and the conductivity of Glycerol, I think it should be in the range of the High Flow Next sensor. If the conductivity was totally out of the sensor's range, all bets are off but in this case, OP was getting reasonable numbers that suddenly changed. I don't know what caused this but I don't think its because OP is using Mayhems X1 coolant.

I think the main concern with using Mayhems X1 coolant with the High Flow Next is Flow Rate - The High Flow Next can be set to DP Ultra or Distilled Water. Glycerol has a much higher viscosity that Ethylene Glycol (945mPA vs ~20mPA @25°C). This will significantly affect the accuracy of the High Flow Next's reported flow rate. Since Glycerol is much thicker than Ethylene Glycol. it will slow the mechanical rotor down resulting in lower flow rate values. As the coolant temp increases, the viscosity decreases, so the deviation in flow rate will decrease.