Well, once more - when you SELL an Item with BOM underneath it, only for this single Item the Qty on stock is decreased.(If you do not “create” them, you’ll simply get negative inventory of them.)
Components (Items in the BOM List) have already been written off when BOM Journal was posted, as I explained previously.
However, if you bypass “creating” your Assembly BOM Items thru BOM Journal, the components will NEVER be written off - the BOM List in Item Card should be treated as a “recipe” only, used by BOM Journal to know which component Items should be written off.