The Pokémon Global Link website has announced that a patch will be coming in December 2017 for Pokémon Ultra Sun and Pokémon Ultra Moon that addresses some issues.

The Pokémon Ultra Sun and Pokémon Ultra Moon Update Is on the Way

An update for Pokémon Ultra Sun and Pokémon Ultra Moon will be available later in December 2017. This update will be required to play online—you will receive a message in your game if you attempt to go online before updating.

Note that once this version has been installed, any Battle Videos saved prior to the update cannot be played or shared.
The update will include fixes to these issues:
· On rare occasions at the beginning of the game, if you choose Litten as your first partner Pokémon, the game stops right after the cutscene and prevents you from proceeding.
· The Wide Guard move doesn’t work correctly against some Z-Moves.
​· The Ion Deluge move doesn’t work correctly.
· When a Pokémon learns moves using BP at Mantine Surf Spots or Battle Tree, it will sometimes automatically forget moves it already knows.

Source: https://3ds.pokemon-gl.com/information/23373794-1b54-4148-b297-706f6a1f715e