Reference: https://us.battle.net/support/en/art...acter-transfer
X-Realm Mythic and TransferOriginally Posted by Blizzard Entertainment
Why?It's extremely disappointing that we had a pug Holy Pally yesterday for our raid that everyone really liked and as a guild we fronted the money for them to server transfer. Then couldn't raid today because their Raid ID was set to something different due to transfer. Even after opening a ticket to have them fix it the request was denied.
So as a warning don't do this if you find a pug that would like to join your guild after doing X-Realm Mythic. Just let them stay on their own server and continue using X-realm as that seems to be what Blizzard is encouraging.