Might have multiple issues, I noticed that your yellowing was mainly at the top, so I thought either the lights were too close or too much potassium the leaves look like they are showing signs of k toxicity.

The soil PH on runoff is low but not as low as you might think I have done that test and even tested the soil as well as the PH and I remember reading that if you are testing runoff you should add a point so 5. would actually be 6.2 and although on the low side still within sweet spot of 6.2-6.8 and always a good idea to fluctuate the ph between those two ranges this allows the plant to pick up all nutrients needed and not just the ones in a particular range. I would be careful increasing the nitrogen at this stage of the plants cycle. Hope this helps ya Sarge