Oh yes I forgot to add that you should also try overexposing the scene before MWB. This is from the Sony Help guide:
- Setting [ITU709(800%)] or [S-Log2] may cause an error in the white balance custom setup. In that case, set the exposure bright first and then perform custom setup.
If you have the time, switch to a B/W PP, overexpose by 1+ stop then MWB, then switch back to S-Log. It's a stupid bug that still hasn't been fixed but hey, it's still gorgeous video.