There is some truth to what you say, but Tatum saw about 8 more double teams a game last season, than he has this season.Is it really fair to compare Tatum's shooting percentages to previous years? For almost his entire career, he has often shared the floor with a third creator in Kyrie, Hayward or Walker. Obviously the effectiveness of those players has varied, but I think we can all agree they were guys that drew attention from opposing defenses and given Tatum better looks. This season he doesn't have either of those guys, and Jaylen has missed significant time as well. He just doesn't have the kind of supporting casts that is going to create more advantageous situations to score than he has throughout his career.
For star offensive players that lack support, I think we need to recalibrate our expectations for efficiency. Some players like Curry are freaks that can maintain their efficiency, but for a lot of players, being the only real star-level player on the court is going to lead to worse efficiency. There are naturally just going to be a lot of possessions where the offense is stagnant, Tatum gets the ball with five seconds left on the shot clock, and he just has to put up a tough shot because nobody else can do anything.
Tatum definitely should be shooting the ball better...but I think some regression in his efficiency can be expected. It can be a frustrating pill to swallow given that our expectations for Tatum are that he should be getting better and better year-over-year given his age and the history of his progression, but there is a big learning curve for him this year I think in playing mostly with more limited role players and not having a Kemba-like player to draw some attention.
A big part of that is Ime has Tatum running PnR far less than last year (a little over 8% less of the time). Tatum is also getting more open catch and shoot threes than he has gotten in the last three seasons. His biggest issue is he’s shooting step back threes at 28% so far, after being between 37-40% the previous two years.