We raise invoices daily for our customers directly from the Invoice Module and experience no issues with stock adjustments.

However today I've noticed that invoices generated from a Sales Order (normally raised from quotes issued), and where we later add additional stock to that invoice (not the SO), that after posting the invoice those stock levels for those items are not adjusted. 

I have since learned that the SO will update stock levels upon despatch (not when the invoice is posted) but fail to see why "new" stock added to the invoice cannot be updated when the invoice is posted as this does not affect the original SO.

Can someone explain the reasoning behind this please?