The Legion Artifact appearance that is rewarded for completing a +15 keystone was not updated/scaled with the new Mythic Keystone system released with Season 4 of Dragonflight. Please scale it to the new system (completing a +5 keystone) as it is significantly more difficult to complete a +15 key now than it was prior to DF Season 4.
Mythic+
The Mythic+ system will have rewards up to level 10, with +2 starting from what would regularly be a +11 in the current Mythic+ system.
A +5 should be as hard as a +15 and +10 should be as hard as a +20 in the current Mythic+ system etc.