Interestingly I ran out of WPI lastnight and resorted to a bag of WPC I still had...noted much the same thing. I had it before bed so can't comment on if it gave me an upset stomach (haven't noticed anything), but it was harder to mix, creamier and didn't taste as nice. I hate standing there and shaking a drink, it drains my energy lol especially after a workout.
I think BN source their WPI and WPC from different places hence the marked difference in taste that really shouldn't happen if they're using the same flavouring and just a different filtering process between the two.