I don’t think you’re doing anything wrong. When you had the water tested, what was the result? Trying to figure out how far over 8 you are.
Did you turn off your water features? You’re not shaking you water sample, are you? Also, when you put the 5 drops in, your not shaking it then either - right? I don’t know how much it would affect the reading, but shaking will increase the PH reading. Probably not enough to matter, but I’m trying to think what it could be.
Post a full set of tests. Maybe there’s something else that’s off that could affect the reading of PH that I’m not aware of.
Are you running your pump through all of this? Again, just trying to think of anything.