You are fine. The ProMix is made to be in the 5.5-6.5 range. I use ProMix HP with mycorrhizal and the last 2 107L compressed bales have been consistent. Plants don't have any problems due to medium, all are strain dependant and nutrient demand. If you started them from seed or early on in the same medium they will adapt and optimize to the conditions while they grow as long it is in the 5.5-6.5 range.
I have tested both bales I got from 2 different locations, a month apart and 2 different dates on the bale (Nov 2016 and march 2017) in Western Canada.
Both bales were sludge tested in RO water 0-1ppm.
5.3 @ 5min
5.6 @ 30 min
6.2 - 6.4 @ 120min
Seems to stabilize at around 6.3
I water in always at 5.5- 5.7 and run off comes out at 6.2-6.4. I use Botanicares Pure Blend Pro organic nutes with a few other supplements of various brands. Thanks for doing the flush test, I have been wanting to do that but never get the time, this just confirms that the medium is engineered to be at this range.