What are you testing your PH with? PH paper, meter, or titration?
If there is no variation and your alkalinity turns out to be low I'd question your PH testing method.
Keep in mind Ph paper and titration tests gets old and inaccurate, meters need to be calibrated.
I ordered a total chlorine test that used test strips once for one of the high schools I set up an RAS for. The test was older than the expiration date (even though it was just sent as new) and sure enough they were getting inaccurate results.
I am using Pond Care testing solutions. Like I mentioned the test only tests to 9.0, maybe there is a fluctuation at mid-day it just didn't show on this test. If it matters NH3/NH4 ia 0, NO2 is 0, and PO4 is 0.
