I've sent in a ticket about this. I got the Santa Kang costume from the Holiday 2020 Mystery Box but the game has not recognised it as such. I have the questline started, the Rigellian Christmas toy workshop and Kang, but as the game has not recognised me receiving the Santa Kang outfit I cannot equip him with it to do the task.
The Santa Kang outfit is also no longer available to be won from the Holiday 2020 Mystery Box, it is gone from there and my collection. You may have the same issue as I do.