To OP, did you calibrate your ph meter before the brew?
Nah your beer will be fine. Don't sweat it. I think you may have overshot your acid addition a bit, and as long as your beer doesn't taste minerally you didn't add too much calcium.
Conversion does suffer outside of a range but the range is quite wide, something like 4.7-5.8 or so. I know commercial and home brewers that intentionally mash at sub-5 ph on very light beers to obtain a crisp, just-short-of-tart finish to their beers.
Essentially the important figure which I don't recall you had there is the mash ph in the first 15-20 minutes after dough-in. This is where a vast majority of the conversion of starches occurs and the most vital time when ph must be at the desired range. Remember also that ph is changing pretty steadily during the mash. The purpose of the calculators like bru'n water is to help determine the mash ph during that key 30 minute time frame at the beginning of the mash. The ph will dive during this first 15 mins then start to stabilize and even rise slightly by the end of the mash, before the sparge.
As a general rule ph values can fluctuate by up to .1 just from instrument error. Add to that most of us are not taking ph readings at the exact same time in our mash nor at the same temperature across batches. So some variance can occur there. Really the only reason to take ph readings of preboil and post boil beer is to have an additional set of data points to identify trends when something begins to be out of whack. That's pro territory there though.
Again your beer will probably turn out just fine. If recommend calibrating your ph meter before each brewday, taking samples at the same time during the mash, and cooling them to 70-80F before measurement. If you are consistent in this, your ph readings will be way more useful for mash and water chemistry purposes.
Happy brewing!
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