I would suggest take a sample ...

I would suggest take a sample of your water after the pH measuring point. In the lab stir it continuously in a beaker and check the pH for the same residence time as the fluid spends in the pipeline. If the pH rises its an ongoing chemical reaction most likely from the pH correction. If not then your problem is most likely leaching of salts from the pipeline materials or corrossion by products. Of course as previously suggested you should ensure that the pH meters at both ends are calibrated correctly.