I’ve noticed this too and I’d like to suggest a different explanation. I ate nothing for 36 hours, except for a couple of coffees with cream; I was cold and hungry and didn’t want my metabolism to slow.
It seems to me that what happens is that after my guts have had nothing to do for 36 hours, peristalsis sets in very quickly as soon as I eat and so flushes out what had been left behind before I started to fast. What comes out cannot possibly be what I have just eaten, because there hasn’t been time for it to be digested