What does "good digestive shape" even mean? I've never heard of "digestive muscle wasting" in any of the medical literature. Your digestive muscles are smooth muscle (different from the heart) that are autonomically innervated (more than just food stimulates them).

The fact that people have gone on extended fasts and then broke them without much trouble suggests a "breakdown" of digestive function is a weak argument.

I would say that a change in the biome of the digestive tract is probably more responsible for any digestive upset after breaking a fast than "muscle breakdown".