That's the right thing for the wrong reason, which can actually significantly forestall figuring out what's actually going on.

Scurvy is like that. They figured out that if you give sailors a ration of fresh fruit juice they don't get scurvy - this works because the fresh fruit juice has Vitamin C and Scurvy is just Vitamin C deficiency. But they didn't know why it worked - why would a cow, a mango, a lettuce, and indeed basically everything else have a chemical in it that humans can't synthesize ? That's crazy. Except, nope, that's really what's going on. At some point an ancestor deleted the synthesis pathway for this chemical, their offspring weren't bothered because fresh literally everything has the chemical anyway, so the deletion stuck around.

At the same time ships get quite a lot faster, travel across the Atlantic is now rapid enough that even if you deliberately abstained from Vitamin C you wouldn't get scurvy, your body hoards many chemicals it needs (a few must be made "fresh" but most can be hoarded) and only develops issues when the hoarded ascorbic acid is exhausted after a few weeks. But they don't know that, and so when they swap a working solution (store fruit, squeeze the juice out, drink fresh juice) for a non-working solution (squeeze the juice, preserve that, drink preserved juice) the apparent results do not change.

Until they do something more extreme. Go to Antarctica, which is incredibly hostile to life anyway, and then stay there for months. Despite your magic preserved fruit juice you get scurvy. The juice wasn't magic, and eventually you will figure out it was the freshness which was magic and from there that there really is a chemical found in cows, mango, lettuce and basically everything alive which we cannot synthesize. Ascorbic acid, Vitamin C.