That is a great looking water report. The alkalinity is nicely low. Hardness is low, but that does not matter.
Given the low alkalinity, I would not be surprised if the mash pH fell into range if the OP was performing a normal mash procedure. Even for lighter colored beer grists. But since the OP says he does the full volume of water when mashing BIAB, it is possible that there isn't enough acidity in light colored grists to bring the mash pH down. Bru'n Water allows you to check that out. It also allows you to check if that 2% amount of acid malt is enough to do the trick for the pH.
Given that the OP says his darker beers seem fine and the lighter ones not so much, it is probably the resulting mash pH that is creating the problem.
BIAB is a nice method, but the high amount of water in the mashing step can accentuate the amount of alkalinity in the mash. On the other hand, given the relatively low alkalinity of this water, if the OP was mashing with a normal water to grist ratio, the mash pH would probably fall too far when brewing dark beers.
Check out Bru'n Water. It is the perfect tool for situations like this.