Most common reason for this is “Automatically update online stock quantities” in WooCommerce settings is not checked on, or “Manage Stock” is not checked on in Sku Info tab.

But your case is probably caused a bug of the recent version. Did the quantity change before your last upgrade(May 12)? If so, would you like to update all stock quantities from POS to web manually, I can do it for you. Otherwise, please give me an example Sku or product name, I can have a look in detail.