I've been trying find whether anyone has commented on this, and that it's the orbital textures being drawn. I have a knack of not putting in the right search words!
My test is: start up at EGCK Caernarfon and set the weather manually to CAVOK preset, but then change visibility to 10 nm. "C" to leave the aircraft, SHIFT-up to gain a little height, then rotate to look at the Snowdonia mountains. You'll see the edge of the orbital scenery at 10 nm. Climb further (takes awhile) and you'll reach a height where that edge starts to march off to the distance.
Others on the Org forums have suggested tweaking min dsf draw distance to 110000 and max to 150000 and it works for me!
Update 28 Aug: or rather it did. I downloaded the latest FlyWithLua and it (quite rightly) prevents access to private art DataRefs. So I mustn't go flying in realistic weather for now.
Is this a bug? If so, I'll look for where to report it, if it hasn't been already.
Incidentally, X-Plane 11 is very good!