...did you ever turn the machine back on without changing the name back on a mastered file...
Yes, and the DP behaves as though no master file had been created and will not go to Master mode. Once the tracks have been mixed, the DP will always look for the {songname}.WAV file when trying to go into Master mode, and that name must be exactly the same as the name of the song as it appears in the Menu screen.
Mixing creates {songname}.WAV.
Mastering renames {songname}.WAV to {songname_z}.WAV
Mastering uses {songname_z}.WAV to create a new, mastered {songname}.WAV file that overwrites the original mixed {songname}.WAV file.
If you press the Re-do button in Master mode, it loads the _z .WAV file (the original mixed file) that can then be re-Mastered, again overwriting the current {songname}.WAV file. Thus the original mixed file is backed up and always available so that the original mix can be "mastered" as many times and ways as you like.
If you go back to Multitrack mode, then to Mixdown mode, and do a new mix, the previously mastered {songname}.WAV file will be overwritten by the the new mix file. If you first rename the previously mastered file to something else, you preserve that master. The DP then will create a new {songname}.WAV file for the new mix, and the process starts all over again when you enter Master mode.
The same is true if you do this with the mixed stereo file created in Mixdown mode. You can preserve copies of multiple mixes by renaming the file, and then choose which one you want to master by changing its name back to the original name and copying back to the SD card, thus overwriting the current mix file on the SD card.