@Peebz
Everybody's water is a little different but it sounds like something is off. To drop a gallon of my water from 6.8 to 6.3 it takes around 3ml of GH ph down
Are you using a syringe to measure?
How are you testing your pH? Drops?strips?pen?
Are you mixing a gallon at a time?
Next is you DO NOT want to use PH down and then ph up on the same water, it can make it toxic. This may be why it is incorrect after feeding.
If I drop the pH a little too far, I add un-ph'ed water to bring it back up, but if it drops to low I start over.
Your plants are not looking like you are having any problems from the past pictures so the last thing you want to do is go changing things up if you don't need to. Don't read to much into your runoff if your plants look healthy.
Just focus on dial in your feeding practice.