You feed similar products as me but I measure everything in ml so its hard for me to quickly see whats up. I have had this same condition before, but it happened only when I had some type of irrigation error and let the plants wilt somewhat severely.
I mix 60 gallon batches but I did 5 gallons before I had the tank in place. I used RO for a while and now use DI. PH can be a challenge especially if you are trying different things. I keep it all in a grid chart log type of thing and that makes it easy to repeat results. I've never not been able to get the PH right. One thing that was interesting is I picked up a second meter for PPM and the they are 75 apart in the 500-600 range. That was a surprise.
In 10 gallons my mix for that stage of the grow is:
Si: 20 ml / 4.25 tsp
(6.5 ml of PH Down to prepare for Cal/Mag. 1 Hour wait before adding)
Cal/Mag: 21 ml / 4.2 tsp
GH Micro: 42 ml / 8.5 tsp
GH Grow: 42 ml / 8.5 tsp
GH Bloom 20 ml / 4.25 tsp
FulPower: 125 ml / 25.3 tsp
YuccaWet: 29 ml / 5.9 tsp
GH RapidStart: 15 ml / 3 tsp
Hydrogen Peroxide 11%: 29 ml / 5.9 tsp
This puts me right at 600ppm and reasonably close - usually - to 6.0ph. I continue to dial the PH in and I rarely vary by more than .05 from 6.0. But a week later there is drift in the tank that has to be addressed. I just started noticing it.
I run the lights 24/7, watering 3 times per 24 hours until run-off.