Not so much a bug, but the way NES Screen Data is handled.
The system can draw as small as 8x8 tiles, but palettes are handled in 16x16 chunks, and the attribute table is handled in 32x32 chunks. Fixing the attribute table would require more code, which is already limited in space.
Eventually I'd hope that they set up the Text Box mode to only allow you to follow the attribute table so people don't run into this bug, but for now the easiest solution is to expand or shrink the weird side by one block to align with the table.