The default abilities for many neutral hostile units were changed to include invulnerability. Many custom maps did not modify this ability because the field was empty and they wanted it empty. As a result, the first wave of many tower defense games are invulnerable (for example: Liquid Tower Defense), for others it is some future wave.
Because changing of default values causes backwards compatibility issues like these, they should be left unchanged or changed conditionally on the version of the editor used to create the map. Then maps made with an older than reforged map editor could have their old defaults intact.