Lee was about equally bad against lefties and righties in 2024. He improved in 2025, to be about equally mediocre against both. This year, he's quite good against righties and back to being bad against lefties.
There's some chance we're seeing small sample sizes (in all of these seasons), so I would hold off on drawing conclusions until this season develops further.
Is there any reason to think Lee would hit better against left-handed pitchers as a left-handed batter? There isn't any data I can look at to decide one way or the other. But the statistics against LHP could be interpreted in a different way than you propose: maybe he simply should not bat against the lefties.
Royce Lewis can platoon at 1B with Clemens, or platoon at 3B with Lee, but not both. (Clemens actually has had marginally more success against left-handers than Lee has, this season, but neither is actually "good.") As one example of a solution.
I'd rather concentrate on making Lee better against lefties than to relegate him to a platoon. And since he's got so much experience as a switch-hitter, I'd be inclined to keep working with him to improve that way, than to throw it all out and start over by making him a full time lefty.
(Weighing against my conclusion: in Lee's only full year in the minors, 2023, his splits revealed a lot worse performance against LHP at Wichita and St Paul. Still doesn't suggest one way or the other how he'd do against lefties if he faced them from the "wrong" batter's box.)