I'd really love to know exactly what the glasses are correcting on Muncy.
If it's a simple case of nearsightedness, and they didn't catch it until this year, LOL at that.
It's possible that it was just slight nearsightedness, and since Muncy was always known for his batting eye, they assumed he was fine, and dismissed it. Then, when he got off to this horrendous start, they said, "Here, try these glasses" and he took off.
It's also possible that his vision substantially worsened and they didn't realize it. Maybe they don't do regular vision tests of existing players?
I guess there's also a chance it's like Dumbo's feather, and the glasses are a placebo, but I'm guessing he really needed them, and that was the reason for how terribly he was hitting.