This is a history of my ferritin levels while taking iron supplements:
OCT 2018: 8 (out of range low)
NOV 2018: Received Infusion (Injectafer 1000 mg)
DEC 2018: 101 (normal range)
MAR 2019: 56 (normal range)
APR 2019: 48 (normal range)
FEB 2020: 20 (out of range low)
Red blood cell count, hemoglobin, hematocrit, etc, were all normal for all of the above tests. For menstruating women this is a more common condition, obviously because of blood loss. However, I am man with no sign of blood 'leakage'. I've had endoscopies, colonoscopies, MRIs, etc.
You mention other conditions that can cause HWA -- could you name some?