Hi, I noticed that quantity value disappears when theme is elementor based. In WPB it is ok.
It is happening in every place where this input is available.
I even tried to adjust font size,color for .quantity input[type=number]
Please deactivate all the plugins not related to the theme and switch the parent theme, check the issue, If the problem has gone, activate the plugins one by one, checking the issue to detect which one causes the problem.
if the problem remains, even after the plugins deactivated leave the plugins deactivated for us to check and provide your site admin access to the private area.