So, the site is still under construction, so we're putting things together little by little.
In the meantime, I've solved the problem by using the "Show Selection" view. That works.
However, I've also seen the problem on another site that uses a different template, so I don't think it's a problem with the template's CSS.
Also, there we fixed the quantity display by changing the CSS from inline-block to inline-flex, and it worked fine. But the number in the center field still doesn't appear.
It's also strange that it appears and works on the category page, but not on the product page and in the cart, as you can see from the screenshots I attached.
We're sticking with the other method. If you happen to think of a solution, please let us know.