I'll explain very concise.
Tatum is a scorer.
He is not a rebounder, versatile defender, glue guy, vet leader, playmaker, swiss army knife nor an outside shooter specialist.
Last year, in our specific circumstances, we missed our 2 main scorers (KI, GH) and Tatum was put in that role, a scorers role. And, since he is a natural scorer he did very well in that role, even as a rookie.
This year under different circumstances we needed Tatum to be a defender and an outside shooter, with his scoring being less of a need.
He didn't deliver in that role since he is not a defender or an above average outside shooter.
Simple as that;
Last year - played his single natural role
This year - as he doesn't know much beside scoring he failed in his unnatural roles.
I'd just add that TT needs a lot of work as his game has many below average elements.
It has nothing to do with one good dunk of his. It has no correlation with healthy logic at all. Also, this is not Brad's mistake, this is Tatum never learning this and not being wired that way, and that's on him.