A tablespoon should have easily adjusted the pH.
I'm assuming you are using a pH meter since you give a specific pH value that you wouldn't get from a test strip. Has it been calibrated lately? In the lab I will always check this before using it. Typically first at pH 7.0, and then either 4 or 10 depending on if I'm measuring something with a low or high pH. I those are spot on, then I'll proceed to my sample. You might double check with another method.
Also, when you heat water the pH will naturally rise. How much I'm not sure. If you are meausring pH with a pH meter, hot water will absolutely give you an innaccurate reading, they are calibrated for 68 F - just like refractometers. On should cool a mash sample before measuring pH (unless you sprung for a model with ATC). pH stips don't have this problem. I've never bothered with checking the pH prior to mash-in so I can't really comment. Maybe someone else will chime in who has measured the pH at a whole bunch of steps. It very well could be that once you added the grains, your pH would have been fine.
You could always make a test sample, say 20X less of both the water and the grains, mash this in as you normally would (150F or whaterver) and check the pH. Just make sure to use the same proportions.
The problem is that you are trying to acidify you water without acid. The 5.2 ph is a mixture of salts that will only work with your water and in the mash itself.
Well no, actually there are plenty of salts that will alter the pH and act as acids (or bases) and do a perfectly fine job of altering pH. The salts in 5.2 will very effectively lower the pH of a solution (or raise if it is too acidic). Granted, an acid might do the job quicker with less material, but it is very easy to overshoot your target pH. That is the nice thing about 5.2 - you can't overshoot.