He's not good against LHP, the OPS+ is hiding the reality.
Last year he had a 103 wRC+ against LHP, yes. But he did so with a .344 BABIP that was very FB% heavy. He saw a significant reduction in hard hits against LHP as well while keeping similar K% and BB%. So the 103 wRC+ is likely inflated by some soft-flyball BABIP luck and is unsustainable. It was also in 90 PA's, likely more curated than most samples...he hit against the lefties the Cubs wanted him to and the tough ones he was shielded against, another reason to believe the 103 wRC+ is inflated.
Secondly he's been terrible against them when he's gotten chances this year, rocking a 47 wRC+. Is that probably affected a bit by small sample, bad luck and playing sparingly? Yeah probably! Meaning the truth of "how well does Busch hit LHP?" is between the 47 wRC+ and the 103 wRC+ last year. It's likely not good enough for him to play against LHP. He's not entirely futile against them, but there's no reason the Cubs should feel the need to play a 90 wRC+ hitter if they don't have to. The Cubs still believe that Turner's the better option.