That seems odd that RO
water with salt freshly mixed is low ph. I would try to rule things out. Can you get another source of water from you local fish store? Or even goto super market and buy a gallon of RO
or distiller water mix salt and see if tests the same. It could be your test kit or your water source is my guess.
Try bringing some of your water to local fish store and have them test. Don't tell them your issue and see if they come back with low ph? Maybe buy a ph pen? Fairly cheap and way more accurate than liquid tests.