RadButton, RadPushButton and RadLinkButton can have a custom icon URL and CSS class.

When a CSS class is added, the background-image: url() rule is not rendered initially.

It does not matter what the class holds, it can even be empty

If you hover and leave the button, the icon shows up because the inline rule gets rendered.

The RadToggleButton does not manifest this behavior

You can find attached below a simple repro and workarounds