Almost every morning have a smoothie with one heaping spoon of vitamin C powder, one heaping teaspoon of sucralose powder (mitigates the pucker factor), one frozen banana, some other frozen fruit, and white grape or apple juice to fill my blender bottle to 16 ounces. On mornings when I'm planning to go to town I have something less likely to cause me bowel problems, but still take the same amount if C in pill form.
When I use grapes, pineapple, raspberries, or strawberries as the non-banana fruit I'm fine. If I use pieces of frozen apple I inevitably exceed bowel tolerance. I have purposely tested to verify this, much to my annoyance
Any reason you can think of why apples would have some kind of synergistic relationship with vitamin C? Apples and C are a dangerous combination, for me anyway.