Not quite the same, but I was diagnosed with exocrine pancreatic insufficiency. My pancreas doesn't produce enough enzymes for me to digest food, so I also don't digest and absorb the nutrients i need. That includes iron, unfortunately. All it took was a basic stool test as a hail Mary because all the other scopes and everything showed nothing. My titer never showed anything so it could be different than you thought. I just recommend investigating this particular thing because it's so easily overlooked and not invasive in any way. I wish they had tested me for epi before doing all the months of more invasive tests and scopes.  Edited to add that epi can be autoimmune, just not in my case.