Hey SE_JonF,
removed previous comment. Figured out some things that removed it from being part of that bug we were looking at.
Edit: Looks like it may be a change happening somewhere. I’ll update when I have found the issue.
Edit 2:
Okay, so this isn’t a bug, but instead a change in the way the materials were handled with 4.4 and before vs how they were handled post 4.4.
Essentially, the color curves were changed significantly in the BP_Sky_Sphere to keep from using negative values. If you take the the sun brightness as an example and change that to a negative value you will see that it becomes a void swallowing up everything around it. In 4.5 and later, if you do the same you will see that this no longer affects the clouds and only makes the sun disc a small black void.
Since the BP_Sky_Sphere is not a part of a project and is an engine asset when you upgraded this part was not brought over from the original project since the engine already had a copy.
This is where the differences come in.
There are two ways to go about using the old one without having to change any values:
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(Not recommended, but doable) You can open the Engine folder for 4.5 or later versions and replace the EngineSky folder where these assets associated with the sky sphere are located with the ones from 4.4. This will fix your problem. BUT this will affect any other projects created with this engine version. They will use this old sky sphere and if/when you upgrade to the next engine release you’ll be in the same situation again with the new and improved BP_Sky_Sphere that is included with the engine. If you choose this route, make sure just to create a sub-folder here and name it backup. Place the newer version of the skysphere in this folder in the event you want to go back.
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(Keeps your project happy without contaminating others you create) Copy the EngineSky folder to your projects content folder and place this in your level with the correct values. This way you’re not using the engine supplied version and since anything in the project folder is brought to any upgraded versions you’ll be able to keep this older version of the BP_Sky_Sphere and it’s contained to this particular project.
Here are my results moving the older version to the newer version of the editor:
**4.4.3
**
**4.5.1
**
Let me know how this works out or if you have any other questions. 
Tim