What about the other two reading times I mentioned? What was the pH normally reading at for this time period and have there been any changes to your routine or the tank?
I would also look into what the saltmix tests at after it's been mixing for at least 12 hrs. If the pH is high there, then the water changes will not help.
It is quite normal for pH to climb throughout the day but it will not usually hit that level until closer to the end of your photoperiod. Be sure you are testing immediately after removing water from the tank as any ambient O2
from allowing the sample to sit will also increase the pH giving you a false reading. If possible also test some RO
water to be sure the kit is accurate.
Be sure you are not testing after any buffer additions or similar chems. You need to wait 12 hrs before testing the pH.