Hello, i had the same issue. I solved it this way, after flashing the rom and getting only 8gb of internal storage i did a factory reseting, the trick that made it work is, when i was doing the tablet setup after the reset, when it prompts you to connect with your Google account you must say that you don't have a google account and you don´t want to create a new one, skip the account setting procedure (you can setup your Google account after in the settings menu). I was driving nuts with that issue and it worked with me twice in a row (haven´t done it after). Give it a try and post feedback please.
I also saw in one forum that it is another ways to deal with this issue with fastboot but it is way harder. Also check if you did a Nandroid backup and that backup is still in the internal storage and not showing up in the file manager.
I hope i helped you guys