Tthe Leica 18mm f/3.8 Super-Elmar-M ASPH offers a brilliant high contrast image, but it is not free of flare.
How does flare behave across the aperture range? This excellent example shows just what to expect when light grazes the front element and results are similar for many of the Leica M wides.
Always shield a lens from non-image-forming light when possible, though there is little room for error with an ultra wide, e.g. a finger into the frame.