Mash pH in check but but high pH post boil.

[beerkench]

I usually get my mash pH to around 5.3-5.4 with gypsum and some lactic acid, checking 10 mins into the mash and calling it good for the day.
I decided however to check on post boil pH after reading that it should be around 5.1-5.2. To my dismay the pH was at 5.6 at room temp.
I calibrate with 5 and 7 before every reading. Other than that I boil for 60 mins using a Braumeister, known for its weak boil, and treat the sparge water although the BM doesn't need much sparge water (I use 2-5l) .
On this batch I also added 1 litre of tap water (pH around 8) to a 25l pre boil to get my gravity in line.

What could be the cause of such an increase when it should be going down?
Is 5.6 too high? Should I be treating again with acid post boil or pre boil? Should I relax?


Thanks.

 

[FastAndy]

Was the mash sample well homogenized? Could be sampling error. what kind of beer is it? A kolsch post boil pH will be much different than a double IPA post boil pH for example.