PH of water/nutes when feeding is 6.5, runoff PH is 5.7 - is this normal?

I have an outdoor grow in cloth containers, soil is Fox Farm Ocean Forest. I’ve just switched from Pure Blend Pro Grow to PBP Bloom. About a week ago I started supplementing the nutes with CALMAG (at the ratio of one teaspoon/gallon) to address what appears to be a Magnesium deficiency.

I’ve been careful to adjust the PH (after adding nutes and CALMAG) to 6.5. For the first time this season, I decided to check the PH of the runoff of a few plants and it was around 5.7. Is this normal? The plants look healthy but I’m concerned I’m doing something wrong.