I believe I've found the reason for this; the last version wraps the export routine in a SketchUp API construct which prevents extraneous undo operations from appearing in the undo list. This issue is apparently due to that. To defeat it, you can open [sketchup]/Plugins/maxwell/lib/export.rb
and "comment out" the following lines:
86 if Env.on_su7?
87 model.start_operation("Export MXS", true, false, false)
89 model.start_operation("Export MXS")
To "comment out" means to place a pound sign in front of the lines to make Ruby ignore them (this is used for putting comments in code, hence the name):
86 #if Env.on_su7?
87 # model.start_operation("Export MXS", true, false, false)
89 # model.start_operation("Export MXS")
Please try doing this and let me know if it takes care of the issue for you -- it does here.