Yes, we see the problem and already described why it happens. The problem is that your custom plugin adds a similar script as in our theme that increases and decreases the value on +- click. And two scripts work at the same time causing double increase/decrease. We can disable our theme’s script if you want. But in this case, our functionality will be disabled completely on all products and work only in places where the plugin loads its script. Otherwise, you can contact plugin developers for help and disable their scripts.