Take this for what it is worth, google AI says that 6.5 is actually a good number. If you have that steady reading while doing a flush that means the coco has reset itself to a stable starting point. I don't know if that is entirely accurate or not, but it sounds logical.
Do you have a way to measure the EC? google AI and another coco specific web site also say that EC is a better measurement because it measures the salt buildup in the runoff. That is accurate.
I grow in a 50/50 blend of coco/perlite. When I wash my used coco I use plain PH'd tap water and then a Ph'd solution of water/cal-mag, but haven't ever measured the PH in the runoff. I usually let the runoff drain away entirely simply because of the volume of water.