jan 29 temp got super low down to 17 celsius that night.
jan 28 replaced dwc water with heavily phed up and down solution.
at this time i got the ppm meter and pg meter in the mail, i had been topping with nute water casuing high ppm so when i found this out i would just top with regular ph'ed water for the week and replace the solution every week. yes i went this whole time without a ppm and just topping with nute water, probably what caused my claw/nute burn
i concur with your diagnosis -- ph fluctuation
if it continues and spreads watch the pattern sometimes light burn starts between the ribs of the leaf like that as yellow and spreads
especially if it gets suspiciously even patterning -- ph spots are usually irregular spots
@DojoGeejer, never used it for that purpose but i am a big fan of potassium silicate in coir, makes much stronger plants
it will definitely make the roots of any of your plants less susceptible to shock from any source so it should work to help against ph shock i suppose?