I can see this going a few ways, and I am not sure which of them would be the best option.
1) Falma/ someone else somehow "invents" new cures to currently incurable illnesses, whether by using a substance only found in that world or by "magic". The downside is that this is basically a cop-out to the problem. It only works because the author directly says it works.
2) Modern medicine actually makes significant breakthroughs for cures to numerous illnesses. The downside is that this is highly unlikely to happen.
3) The story becomes vague about how it handles certain illnesses that there isn't a known cure for. It maybe addresses them as a passing issue like, "we have so and so working on finding a cure, because I can't be the one to always do everything for this world".
Or maybe it even puts the issue into the "(seemingly) forever in testing" phase (just like the real world sometimes finds itself stuck in), but that's it. It never directly cures or resolves them, and just ignores them once it moves past them as there isn't an actual solution as of yet. Its basically acknowledging their existence, but sets them to the hardest difficulty setting, making them "so challenging, that even current modern medicine and even magic struggles to properly handle them".
The downside is that this becomes an impassable wall that the plot must always work around. It would make certain things a bit awkward going forward too.
4) The story distances itself from solving medical problems entirely. Maybe introduces a war or a huge conflict to take the focus away from solving current medical mysteries, and instead focuses on doing various, more advanced first aid related stuff. Or maybe it decides to go deeper into the religious aspect and deepen that lore? The downside is that it would be a big change in direction from the current story path and would only be a temporary distraction, like a side quest.
5) The story ends with an optimistic yet empty resolution. Falma looks off into the distance towards the sky and says something generic like "It may take us many years to finally figure it out. The road will be hard and full of trials, but I am sure that if we all try our best, will be able to one day accomplish it..."
The downside is that this makes for a really terrible ending, akin to a story getting axed way too early for a proper ending to be made. It would be like One Piece suddenly ending with "...and after several more years of searching, making countless new friends and having numerous adventures, Luffy and crew eventually found the One Piece."