As food transits past the duodenum, there is still an opportunity for iron absorption but less so as intestinal peristalsis carries the digested food forward. Surgical bypass of the duodenum also prevents physical contact of dietary iron with the duodenum to absorb iron as well. This is applicable to patients who have undergone Roux-en-Y gastric bypass for weight loss. Those patients should have a blood count and iron studies checked once or twice a year at a minimum and consider intravenous supplementation if dietary and oral iron is inadequate.
https://irritablebowelsyndrome.net/living/iron-deficiency-anemia
This is part of the reason why I H-A-T-E gastric bypass surgeries. You are literally surgically making your body less able to unable to absorb certain nutrients, possibly having to take supplements all your life. And in this case, you can’t even take supplements, because you are physically incapable of absorbing them. Nutrients you need to stay alive. It is SO unethical.