Please check on the "look" of your corals before doing radical pH compensation. If they look "happy" don't do anything drastic...my vote is bum pH reading, assuming this is a sudden event.
Pinpoint meters are famous for wild readings and interference especially with the plug-in adapter...use the 9 volt battery see if that helps...I have one tank that when the PC lights are on I can't get a good pH to save my soul.
Also how old is your probe they only have a apx 1.5 year halflife.
If you can borrow another pH meter (or probe) ,do so and compare...not just the readings but the response also....mine tends to overshoot the high pH so it says 8.6 and up some times....unplug, recal (with fresh calibrators) and all is usually well.
Your dKH is high enough to buffer normal day/ night C02 effect to a change of less than 0.5 pH much more is harmful (so they say
).
How high does the pH get in midafternoon? (near the end of MH "day")
Good luck
PS if you are worried turn the lights back on tonightand see if the pH starts to normalize