Yes, but those stock issues are insignificant compared to the average cost issues you are creating.
But that is the exact ramification of what your editing of the Purchase Invoice is doing.
Let me explain with these examples.
You have received two purchases invoices for coconut oil but only one has been edited to Kg:
The coconut oil inventory item looks like this:
Both the Qty on hand and the Avg Cost figures represents a mixture of Kg and L.
So if you happen to use the coconut oil in a production prior to the 2nd purchase invoice being edited, then the production order will transfer at a hybrid cost of 9.22 / Kg.
However, if both purchase invoices had been edited:
So that the coconut oil inventory item now looks like this:
Then the Qty on hand and the Avg Cost figures now represent the same unit - Kg and the production order would have transferred at the actual cost of 10.07 / Kg.
Therefore, your worries about making it harder to determine the raw material cost of the product is justified but that should be focussed on the potential of screwing up the average cost and subsequent product costing, not the inventory item Qty balance.
I agree / support that creating duplicate inventory items and using production orders is a duplication, but so is entering a purchase invoice as L and subsequently editing it to Kg.
Therefore my suggestion is this - enter the purchase invoice ONCE. Review the past 10 purchase invoices for any given product, coconut oil, sunflower oil etc and create an average Kg benchmark for that product. Still use the custom field as this records the suppliers invoice Qty.
This will avoid the problem of a purchase invoice being potentially not converted and if an inventory item needs a “small” Qty adjustment each six months, then this would be inconsequential in the overall scheme of things, .