backward focus traversal gets stuck in button group

If focus traversal to buttons is enabled (using Full Keyboard Access on macOS), then a backwards traversal (shift-TAB) can get stuck at a JToggleButton in a button group.

To demonstrate using the attached test program:
When started, the first text field should be focused.
Type TAB to transfer focus to the second button.
Type TAB to transfer focus to the last text field.
Type Shift-TAB to transfer focus to the second button.
Type Shift-TAB.
At this point, the focus should be transferred to the first text field, but instead it remains at the second button.