moving half a block backwards:

Rotating the camera didn't change anything in both places
It's on max distance (placing the landmark at the sand blocks position won't allow connecting them)

No this is just part of the way the marker system has worked. I'm completely revamping it for 8.0.x though so it should be fixed then.

slaps forehead

Oh right, there are 3 methods that control if a tile should render, isGlobalRenderer which ignores chunk boundries, getRenderBoundingBox which is the biggest AABB that it contains, and getMaxRenderDistanceSquared which defaults to 4096 (64*64).

Sort-of fixed by now, although it doesn't work properly if both land marks are unloaded when looking at them.