Comparing his 2023 to 2024, one thing that jumps out at me is his success when putting the ball in play on the first pitch. In 2023 that OPS was 1.452, and in 2023 it was .709. Across the league it's typical to elevate your OPS on those first-pitch swings (.905 vs an overall .711 in 2024), but for Julien it was extreme on the plus side in his first year and then it was more than gone the second year. Now, this is statistically lacking in significance for an individual player, with 43 and 33 PA to work with respectively in the case of Julien. And another poster noted his elevated BABIP in 2023. Still, small-sample data can be in support of the eye-test, which in the case of Julien is an almost pathological lack of aggressiveness at the plate.
There's a similar eye-test regarding his defense at 2B, which by my recollection is he did okay when the play was within his range and needed to be quick, but if he "had time to think about it" the throw could go astray.
I'm not fond of applying the term head-case very frequently, especially because only an insider can make that judgment, but for Julien I think there's a chance that a specialist in sports psychology could do as much good as any of the coaches in their specific skill domains.