The stock-UOM qty is rounded in _accumulate_so_items, but the purchase
UOM conversion divided it by the conversion factor without re-rounding,
storing values like 5738748.300863984 in mr_items.quantity. The raw
value flowed into Material Request qty and the raw materials CSV, and
make_material_request compares quantity to requested_qty with exact
float equality, so any rounding downstream left dust quantities.
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