Even if you manage to send the quantity on your "Shopping Cart View" the data will be ignored... Quantity only applies to hits with the purchase event.
It's possible that Launch (AppMeasurement) is removing it during the beacon bundling because it's not a valid field in this context (but that seems unlikely).... Same with Product Price... these are only relevant on purchases... this is the data that builds your revenue metric...
if you have a product at $5.00 and the quantity is 3, then your revenue will increment by $15 (3 x 5). On anything other than a purchase you will be counting money as made that hasn't been made, and then when you send it on purchase, you will be double/triple/etc over counting the revenue depending on how many times the user opens their cart (which is why Adobe will just ignore these values during processing, regardless of if they are sent or not)
That said, if only quantify is undefined then you might want to check your Product Quantity data element... but again, fixing this context is a moot point....
For more information about Products, please see https://experienceleague.adobe.com/docs/analytics/implementation/vars/page-vars/products.html?lang=en
Now, if you really want to see interim values for quantity and how much money is sitting in the cart, then I suggest you set up some merchandising eVars for this purpose... and if your Data Element isn't working, you will still need to investigate what is causing it to fail.
I would start by opening your cart, opening your console and running this:
_satellite.getVar('Product Quantity');
Then you will be able to see what this data element is returning, then start digging into debugging your data element.