MyiSAM/Aria traditionally overwrite exiting MYI/MYD/MAI/MAD files when creating a new table.
This, kind of, made sense, because before
MDEV-11412 there was no way to remove stale MyISAM/Aria files.
But temporary tables use generated file names that are supposed to prevent filename collisions. They should not overwrite files on collisions, because there should be no collisions. And if a conflict does happen, it usually indicates that something fishy is going on and an error message would be safer than any destructive action.
This bug was reported by RACK911 LABS