i made a flat land world (1x7,250x87,5x11) and i also tried with 5x10.
when i build in the lava i cant build at the top layer i can put down four blocks from the netherrack, and i can build to anything around but not the top layer.
MC-3105
Cannot build on topmost layer
Resolved
Works As Intended
0
It's intended that you can't build on the highest possible layer.