Tatum's regression to me came on defense. Lets just call it like it was, his defensive effort was lousy in many occasions. The hustle and effort we saw last year on that end which made us think he could be a Pierce/PG13-esque 2-way player was just not there this season.
Overall though, I'd say it wasn't a big regression when you consider he still put up solid stats, he just didn't seem to make a leap that many folks expected.
Also, while Kyrie was bad in the Bucks series, so was Tatum in all but like 2 games. Jaylen outplayed him in the playoffs which I don't think many expected. Certainly not me.