b-ref.com splits page for the player provides another, related point of view. If you divide up his plate appearances between situations where the game is within 4 runs, or not within 4 runs, he racks up immensely more OPS value in the latter instances.
Across the majors you might see 4-5% better OPS during garbage time, and that's sort of intuitive. It's not typical for a player to be 50% better.
Almost all of Wallner's above-average OPS is built up when the game is more or less decided already. Without those PA he has a below-average OPS.
It's statistically amazing, really. He's totaled just under 1000 PA by now, and the garbage time is basically 15%, but it's so off-the-charts that it moves up his overall numbers misleadingly. WPA is the only commonly cited stat I can think of that tips us off to look deeper into his splits.
I'm not a hater. I'm a rooter. I'm rooting for Matt Wallner (and his coaches) to figure this out and make the majority of his appearances productive.