Not a bug
Mr. Fusion 3 weeks ago in Graphics/Visual Performance • updated by Tyler Owen (Lead Developer) 2 weeks ago 1

The Sun lens flare is visible (and appears to be at full brightness) even when the Sun itself isn't (unless that small bright spot where the flare originates from is the Sun, but I'm not convinced it should be visible like that at / near the densest part of a storm) due to the density / darkness of a storm.

It's also worth noting that the Sun is still fully reflected on sufficiently reflective surfaces (such as the porthole window and other parts of the airlock door) even when it's not visible at all on the sky due to the dust storm effects fully covering it. It also still casts rather distinct shadows, and the terrain shadow transition (when the Sun goes below a mountain range and the shadow line moves across the terrain) is also much more noticeable than what would feel "right" with the Sun being fully occluded by all the dust.

Not a bug

I've made sure that the lens flare should now be completely invisible when you are near the center of the storm.

Unfortunately the other aspects that you mentioned are more dependent on the style of rendering I'm using for the day/night cycle. It's very difficult to find a realistic feeling transition to less sunlight during a storm. I've tried to lower the sunlight even more, but there is no reliable way for me to make the shadows more diffuse. Blurred and indistinct shadows are very difficult to achieve. Certainly something I can look into more in the future.