Yes, this seems to be to do with the terrain elevation. The visual airspace stack in What's Here starts (altitude-wise) just below the terrain elevation, so you see a certain amount of vertical distance above that.

I thought that might be the case, but it does it elswhere too (eg just North of Stoke-on-Trent) inside or outside of the AIAA where the ground is the same elevation. Beats me!

I don’t know the reason, but it does the same in that area on a phone in landscape mode. It seems to be related to the surface elevation at a particular point.