Post boil Ph much higher than mash ph?

Hey all did a mini mash of 100% pils malt with RO water and added lactic acid to get the ph down to 4.02 to make some sauergut


Mash was fine
Ph meter was calibrated about 3 times to make sure
Tested it mash about 4 times always came out to 4.01-4.03 ph

did a quick 5 min boil

after the boil I tested the ph again and it had risen to 4.8

how the hell did this happen? I’m racking my brain