Shout out to Intuit Technical Support expert Dave for helping me solve an obscure bug in the QuickBooks Online Plus Edition Inventory.
If you want to add a new inventory item, and adjust the quantity on hand, do this process in two steps. 1. First, create your new inventory item with cost and price, and save it. 2. Second, come back into your Product/Services List and adjust your inventory quantity on hand. Otherwise, if you add and adjust in one fell swoop, inventory will be saved with zero cost. When you go to sell, zero cost will be moved into cost of goods sold, so your COGS value will be too low, and your Profit and Loss Report will not be accurate.