If your overactive immune system results in worsening Crohn's symptoms, then fasting would improve your symptoms (by reducing the immune system temporarily + the benefit of no irritants in the digestive system). When the immune system returns to a "normal" (or normal-for-you) state afterwards, you also have irritants back in your digestive system, making it impossible to separate.

The question is: do your Crohn's symptoms improve concurrently with the duration of the fast, extend beyond it, or track as expected with the presence of food in your digestive system?

I guess where I'm going is that there may be two variables at play that can't be easily separated in the case of Crohn's.