Missing walls when tunneling near edge of map

When I started digging a tunnel parallel to the edge of the map I noticed one of the walls missing its texture. I can walk through it and fall to the void. This appears to only happen when the tunnel is closer than 30 blocks from the edge. Tunnels further away have correct walls on all sides.

I have attached a screenshot for clarification. The map border is behind me. The tunnel on the left side of the picture is less than 30 blocks from the edge and missing its right wall. The tunnel on the right is more than 30 blocks from the edge and all walls are ok.