I was poking around some older Nexus 7 f2fs threads and I came across a couple things:
http://xdaforums.com/showthread.php?p=51521754&highlight=f2fs#post51521754
from @
LiquidSolstice
"I figured out what the problem was. It's a lot simpler than what you describe.*
The reason why I (and I imagine, you as well as the others) are running into this issue has to do with how we're reading the instructions. When the OP says "Format Data" and not to use the "Wipe/Factory Reset", I assumed he was referring to wiping the partitions through the Advanced Wipe menu.
What he's actually referring to is the Format Data button to the right of "Advanced Wipe". The one where you have to manually type "yes" to properly format your storage. That's where I went wrong, and I hope that helps you out too."
Apparently using the normal wipe only wipes /data, but skips /data/media.
http://www.xda-developers.com/android/f2fs-put-to-the-test-against-ext4/#comment-1312611581
Also from @
LiquidSolstice
" The encryption bug because you need to convert the entire data partition to F2FS, not just /data, because formatting /data in TWRP will always skip /data/media*."
nexus 7 4.4.4 f2fs