Autosave has worked fine for me with other files. I'm using Maya 2017.5. I'm guessing this particular file is causing problems because at some point, after a crash, I copied an autosaved version to restore my data. I looked inside the ascii *.ma file but didn't see any lines that jumped out at me like, say, "autosave=true" to delete so I'm not sure WHY Maya is convinced this is an autosave file.