1. Select File > New File or Project > Files and Classes > Qt Quick Controls > Custom Button > Choose to create a custom button.
2. Change the button properties in the different button states.
3. Use the button in another file.
4. Run the project and hover over the button.
5. A box and a border are displayed on top of the button, instead of the correct style.

According to mitch_curtis, this is caused by native styles not playing nicely with custom controls and we should use a non-native style as the starting point for building the control or allow users to pick the style in the wizard.