title_body: What is the reason that Aeropress coffee is more digestible?
upvoted_answer: The digestive problems you've experienced may be due to the acidity of the coffee. Now lighter roasted coffee will definitely be more acidic so the roast level of the bean can be at play. Not only that, but underextraction can cause high levels of acidity also.
  
  The only discernible difference I can garner from what you've told me is that in the Aeropress there is a slight form of full immersion brewing. This will better encourage all parts of the coffee to be well extracted and so reduce the acidity of the resulting cup.
  
  If you're curious as to what the real cause is, buy two different batches of beans, one medium-dark roast and one light roast. Then perform Aeropress, French press, pourover, espresso and possibly drip extractions with both beans and note down your gastric reaction to each cup. If there is almost no significant difference between any of them to how it makes you feel i.e. it seems to occur at random, you might just be hyper-sensitive to small changes in acidity caused by the differences in the imperfections of each extraction.
  
  Good luck.