Aunty, on that A8 probe, have you scrubbed the tip well? Any tarnishing or corrosion on the metals will muck up readings,.. that said, it's possible it's gone off calibration...
-- suspect! For sure those little beauties would be barking about the sour soil if it was that acidic! I can't think of any way to reliably test it out, short of getting a pro' model of soil pH probe,... try this a slurry test.. you have a pH meter in good shape? get some of your soil, exactly as prepped in the pots; make a small pot up for the A8, and moisten it well, but not wet, nor any kind of dry (this will make for poor readings, usually more acidic)..... then make a slurry of 1 cup soil, 1cup low ppm water, let it sit for an hour... test them both and see what the A8 and meter say about the soil itself... at least you might be able to see if the A8 is badly off,...