I fixed it... sorta. I changed the map dimensions to 36x35 and that made the problem go away. I vaguely remember reading on CFC that Civ needs map dimensions to be even or multiples of 4. 34x35 still showed the same problem but 36x35 came out clean.
(I duplicated the rightmost column of the map: in the file, I copied the entire section of plots where x=34 and changed them all to x=35. In the BeginMap section, changed grid width=36 and num plots written=1260. Also, strangely Civ wouldn't pick up those changes by loading a new worldbuilder scenario, even from the main menu. I had to quit Civ entirely to load it with the changes.)
Of course, that screws up the symmetry, you can't evenly divide 5 players into 36 tiles. The best solution might be to add one more column for each player, making the map 40 tiles wide and still evenly spaced. Not a small amount of work but might be worthwhile to have a nice clean map.
(I duplicated the rightmost column of the map: in the file, I copied the entire section of plots where x=34 and changed them all to x=35. In the BeginMap section, changed grid width=36 and num plots written=1260. Also, strangely Civ wouldn't pick up those changes by loading a new worldbuilder scenario, even from the main menu. I had to quit Civ entirely to load it with the changes.)
Of course, that screws up the symmetry, you can't evenly divide 5 players into 36 tiles. The best solution might be to add one more column for each player, making the map 40 tiles wide and still evenly spaced. Not a small amount of work but might be worthwhile to have a nice clean map.