Checkbox fields currently do not indicate, if they have the keyboard focus. They do actually have the focus, but there's not visual indicator.

Please try to show our blue outline around the checkbox itself (see attached example). Our checkbox are kind of small, so it may turn out that this is not enough and we have to move the highlight a bit further out.

While you're at it, please also verify if a checkbox also captures the keyboard focus, if it's the first field on a tab panel the user just switched to (see MGNLUI-2875).