OK. Ferrous bisglycinate chelate is non-heme iron, not heme iron. So that eliminates that oddity I couldn't figure out.
Seeing your labs before iron supplements makes me wonder if you have an a-globin gene mutation. It would explain why your MCV and MCH are low while not taking iron and why, when taking iron, you build ferritin taking iron in a pattern where you would be expected to build hemoglobin much faster. (The other explanation i was thinking would be a transient rise in hepcidin from illness; IL-6 in an inflammatory state raises hepcidin. IOW ferritin goes up when you're sick, and HGB stays the same or climbs slower).
Curious. Thanks for the labs in this. It's a mystery i can't readily explain unless you ever get a hemoglobin electropheresis test (to look for (a) thalassemia gene(s)), which only a very inquisitive hematologist would ever order.