Had the EXACT same problem. Soaked 4” cubes for several hours in 5.5 ph quarter strength nutes so thought I’d prepped properly. Using them in bottom flood tubes with nutes at 5.8. Jacks 321, RO water with CaliMagic and Mammoth P. Reservoir PH stayed relatively stable but the nutrient deficiencies just kept getting worse. Finally pulled a plant and squeezed some solution from the cube and tested. SIX POINT NINE. Yup 6 point friggin 9! Never had this problem in the (distant) past using the same technique. Growmau5 said that the recco from Grodan has changed, they now recco soaking TWICE prior to using. The fact that I’m irrigating only an inch or so and from the bottom kept the cubes from self flushing hence the rise in cube PH without the subsequent increase in the res PH. With proper cube prep my method should be back on track. Thanks to all in this string for their perspectives and input, great thread!