I created a new “time of day” scene, and edited it a bit (but not editing the skydome)
I then turned off ray tracing, and re-started.
Now I’m seeing this:
After doing that, I removed the sky dome mesh, added the BP_SkyDome from EngineContent, kept the volumetric clouds, and then the clouds look like this:
What’s up with these skies?
What am I supposed to do to just get a plain vanilla sky that’s not weird? This used to Just Work in older versions.
I encountered the same thing https://youtube.com/watch?v=EM7mSG6ts5c
I’m hoping these problems are because it’s all great in the 5.0 branch and back-porting to 4.26 made some misses.
But that’s probably me being overly optimistic 
The bottom screenshot has a seam in the clouds because the texture doesn’t tile, just change the texture if you want to fix it.
I wouldn’t bother. Just make your own material, you’ll get much better results.
That is the “new, advanced!” volumetric cloud simulation from the engine, not a tiling texture.
Contrary to “it’s easy!” I find that making good sky textures that integrate well with the lighting and atmospherics, is some amount of work, and trying to do it procedurally with noise quickly eats into fill rate, because the sky is big on the screen.
It’s a volumetric material that uses a (non-)tiling texture, open the material up and find the texture. As I recall it is using the red channel for the primary cloud mask, that’s the channel that doesn’t tile properly.
The material that comes with the engine is fine to learn from but you’re going to have a hard time getting good results from it. You’re welcome to try but the current reality is that there isn’t really an included material you can get good results from by just throwing it into the map and tweaking a few material params. I’m not sure if this was intentional on epics part, perhaps they want to force users to make their own materials to avoid having tons of UE4 games all using the default cloud material.
If you would like some better looking materials then you should enable the volumetrics plugin and check out the sample cloud maps. Just don’t expect density painting to work as that seems to be broken as well atm.
