So in tBBS, Mickey knows who master xehanort was, and knows Terra disappears after what happens in the keyblade graveyard. So why is it in in between khbbs and kh2, Mickey visit's Ansem the Wise in hollow bastion at some point in time and NOT be like "WAIT. HIS NAME IS XEHANORT. AND HE LOOKS LIKE TERRA WITH WHITE HAIR. hmmmmmmmmmmmmm."???? just knowing that the apprentice's name was xehanort should've worried mickey
Mickey never met Terra, nor even saw him at all IIRC And whether or not he knew Xehanort was at the climax at the end of BBS is left up in the air, I'm not even sure he knew what happened to TAV at all
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Mickey doesnt really see xehanort in BBS, so maybe the name got him to do a little digging.
Most people in reality also wouldnt recognize Terra if he changed his skin color, eye color, and hair color. That'd be like if your best friend underwent Plastic surgery and then you had to find him in a crowd. even with the little similarities, the huge differences would be enough to throw anyone off.
That's one theory. the other is that Mickey also didnt see Terra throughout the game at all.