TWRP backup disappeared??

I've been trying to weasel my way into a stable marshmallow rom, or even stock updated ota. So earlier today I did a backup with twrp before I tried to flash (clean wipe first of course) cm13. I tried to restore the backup, after it was successfully completed and the restoration process only took 47 seconds.. obviously something went wrong with it. So I took out the battery, rebooted it and tried to restore it again to find the backup was gone. Same directory though I also checked the external sd. I mean thats not even a big deal bc I have another backup, but does anyone know why a backup would just disappear?

Thanks and happy flashing!

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