There is a bug with the Blizzard API where after you transfer to a new server or rename your character that new Mythic+ runs may get registered to your old character name/server rather than your new name/server. When this happens the data will sit on that old character in "limbo", because the character no longer exists in the API where we are able to request it.
We have a workaround that we plan to implement that will help mitigate these problems until Blizzard can implement a proper fix.