Make sure there are no gaps and no overlaps between the top and bottom heights of the walls.
The surface must line up perfectly (which it looks like they do) and the surface material must be the same (which it looks like they are).
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10