Magento 2 stock inventory is not being updated

check your stock setting in admin panel Advance Inventory->manage stock set to yes if you want to reduce quantity

Check below settings.

Admin > Store > Configuration > Catalog > Inventory > Stock option > Decrease Stock When Order is Placed Set it to yes.

If you have a multi-website or multi-store then change the same setting for all store.

It seems you guys have been working on 2.3.0, where the default QTY will be reduced upon shipment.

M2.3.0 has released a feature know as Salable QTY which will be reduced upon placing an order.