I felt bad for everyone since there is no solution online anywhere so I made an account just to post this solution.
Ok, so I had the same issue, Internal Storage on my device was getting somewhat full. I had about 6 GB full of 11 GB on my internal storage so nothing critical, but I have almost 20 GB free on my sd card formatted as internal storage. I have migrated before and it was always just really easy and at the end I had more space on the phone's internal storage.
...so I tapped on "migrate storage" from the internal storage menu. Migration failed, and my internal storage filled up. WTF?!!!? So I came here and found no solution. I tried some file explorer apps. I could not find where the extra 4 GB of data was that was filling my internal storage. I figured it was some temp file and since I didn't have enough storage to complete the temp file, when it failed migrating there was some file or files that just got "stuck".
I tried migrating again and while migrating I looked at my internal storage. It was back down to about 6 GB of 11 GB full. I figured I could live with that, and I figured the migration would fail again with my internal storage getting filled with some temp file or files. I decided I'd kill the migration right there. With no way to stop it or kill the process that I could see, I just took the chance and turned off my phone. When I turned it back on, the migration process was stopped, and my internal storage had stayed at 6 GB of 11 GB like before.
I guess I can't migrate anymore.... Maybe internal storage has to have as much space free as it has used space make a complete copy of internal storage before deciding what to migrate. This appears to be a flaw in Android. Hopefully they fix this soon. I am running android 7.0 on a motorola g4 with 16 GB external storage and a 32 GB SD card.
SOLUTION THAT WORKED FOR ME: Try this at your own risk!
1. Start migrating again.
2. Once android starts migrating, watch your internal space.
3. Once it goes back down to the previous level, turn off your phone.
4. Restart your phone.
5. You should have that temp file gone and your previous space back!
Not a complete solution because I'm sure we'd all like to actually migrate any possible data to sd card, but at least it gets the phone back to before the attempted and failed migration.