I am not familar yet with the production process in Tryton, but why is 1/2 of productB lost and not left in stock to use in the next production? It seems the production process depends on what productB actually is, and for some products 1/2 will be lost, but for other products the 1/2 remaining can be kept for the next production. Instead of rounding, could the solution lie in identifying the minimum order quantity for the input products, and whether using fractional units is allowed?
Well using a unit of measure that allow 0.5 quantity is a way to manage that if it makes sense. But sometimes it makes no sense to have ½ unit.
In general Tryton does not enforce the conservation of mass in a production. Tryton provides just a proposal for the result based on the BoM but it is up to the user to update with the actual result.