When you originally create an assembly item in AO, it is set up as an Assembly item group showing the bill of materials. However when it syncs to QB, you only see it listed as an Inventory Part. The components are not listed.
When you create the assembly item in QB, then sync to AO, AO will only keep track of the components in AO, not QB. Therefore, when updating components in the assembly item, AO does not update components of an assembly in QB. It is meant for you to keep track of your bill of materials only in AO. Although the components will not update, the correct cost information will sync over to QB.