Hi.
I'm using different theme but also from ThemeVolty. In my case quantity disappears if scrolling if i have a "Product page bottom bar sticky status " disabled in theme settings. Enabling it solved the problem.That's a marvelous hindu coding standards...
I also have a problem with the product photo gallery in the mobile view. It stops working if I disable the blog module from ThemeVolty.
The module must therefore be enabled even if it is not in use.
The connection between these things is beyond my ability to reason...