I've had this happen each time, one time using Advanced Nutrients Sensi PH Perfect; dissatisfied with the results, so I switched over the GH Floranova for my last go at DWC and I got fantastic results, however, my ph still rose during veggy and dropped, ever so slightly, in flower. Maybe .1-.2 a day. I found that if I backed off the heavy feeding, the pH would drop only around .1 every other day though...I believe I encounter this in flower because this is the highest ppm stage of the plant's life. I don't think it's nute specific tho...happened with both AN and GH.