I'm not a plant expert but do keep a lot of discus so I know a thing or two about this problem. I would think that you have hard water in your area. If you plan to adjust pH by mixing with RO water, you need to mix more RO water than tap. I use 75-80% of RO water. Mixing 50/50 doesn't mean you would get something right at the middle. It all depends on your water hardness.
Also, as Murphy said, you might have something buffering your tank. It's very unlikely that after the 50/50 mix, the pH would jump higher than the original point (even over the tap water pH). What substrate are you using?