This is an RMXP glitch. Every once and awhile this will happen and it is impossible to fix. You cannot solve it.
The way I fixed this glitch in my first game(Which got deleted right when I was going to make a backup), was by making a different map with a different name that looks slightly different.
In 2K3, there are a LOT more glitches with tilesets such as-
-The same as above, except no tiles in the tileset will NEVER work, no matter what map or name.
-Random map deleting(A real pain, especially when its a huge map with an incredibly long event in it).
-The game crashing whenever a monster tries to heal itself.
-And much more.
Sorry about your problem, but it has happened to me before too.