Immediate Mode: Draw.RectangleBorder invisible while Draw.Rectangle with same parameters shows

When I try to replace a Draw.Rectangle with Draw.RectangleBorder the latter seems to not show.

This shows:
Draw.Rectangle(
 new Vector3(0,0,0), // pos
 Quaternion.Euler(90, 0, 0), // rotation to face towards y axis

  0.5f, 0.5f, // Width, Height

  Color.magenta // Color
);

This is not visualizing (same parameters as above just adding thickness 0.1f):

Draw.RectangleBorder(
new Vector3(0,0,0), // pos
Quaternion.Euler(90, 0, 0), // rotation to face towards y axis

0.5f, 0.5f, // Width, Height
0.1f, // Thickness

Color.magenta // Color
);

I couldn't figure out why this is and have to assume it is a bug, unless I understand something wrong.
Any help to make rectangle borders work for me, would be much appreciated.