Does water entering the anus during istinja', even if doubtful, invalidate the fast according to the Shafi'i school and necessitate its repetition?

This doubt is from the waswas (whispers of Satan) that must be ignored. The fundamental principle is the validity of your fast unless it is definitively proven that something entered the anus. The original state remains as it was, and certainty is not removed by doubt. The view that the fast is not invalidated by something entering the cavity through the anus is a strong opinion, well-supported by evidence.