Hit the post button a bit too early earlier. Mobile SB is fun lol.
Anyways, people need to keep in mind that with Smash, the characters between all phases weren't chosen at the same time.
Pass 2 was probably greenlight in response to the success of the first pass/Ultimate as a whole, and thus probably planned much later. And Min Min's status as a spirit upgrade has proven that they are willing to jump back on characters they already had roles for.
What I'm saying is Rex's status as a mii costume was probably added during a time where pass 1 was being planned as the only pass, and was seen as a way to throw a little bone to his fans as a little "honorable mention" of sorts. Now that we do have a pass 2, and with fellow base game-absentee ARMS finnaly getting it's chance with Min Min, I can totally see em going back on Rex, regardless of his status as a pass exclusive Mii Costume