calculate_item_values rounds every Float field on an item row to the
site's Float Precision (3 by default), and conversion_factor was one of
them. The factor is a ratio, not a rate: UOM Conversion Factor.value is
stored at precision 9, and Material Request keeps the full value because
it has no currency field and so never runs the calculation.
Mapping a Material Request to a Purchase Order therefore truncated the
factor - 0.453592292 for Pound -> Kg became 0.454 - and stock_qty, which
is recomputed as qty * conversion_factor, drifted from the quantity that
was requested, leaving the Material Request unable to close.
Exclude conversion_factor from the rounded fields on the server and on
the client. Factors below the site precision would otherwise round to
zero outright.