Blizzard
The changes we're making to the Mythic+ system for Battle for Azeroth will prevent you from receiving your normal Mythic Cache this week. As this was unintentional, your cache will include 1000 Wakening Essence and a Keystone instead of your normal rewards.
If you have a problem looting the Mythic Cache, you'll receive the Keystone in the mail and the Wakening Essences will be granted after you log back in.