Huzzah!
This seems to be a reoccuring question. Some people were able to get their Mixamo rig to work by renaming all the bones in the mixamo rig to ones that match Epic’s rig. This is the best answer I have for you.
THIRDPERSON DISABLED = ARMS INCORRECTLY VISIBLE BUG FIX
This will be included in the next update, however if you want a fix for this now, add the following nodes in your BP_PlayerCharacter event graph.
