Warning! Long and complicated explanation:
I'm re-reading "Ecology of the Planted Aquarium," which is very good if anyone is ready to dive into some dense science. D. Walstad is the author. Anyhow, I'm on the chapter about allelopathy right now.
Plants and algae both produce a wide variety of chemicals to deter things that want to eat them, or to kill other plant and algae that compete with them for space and nutrients. That's why it can be hard to add a new species of plant to a tank that's already been heavily planted with one or two other species. The first species of plant will have had time to release chemicals into the substrate and/or water to inhibit growth of competing species. The new thing you plant doesn't have a chance to get established and start pumping out it's own chemical warfare before it is poisoned.
Algae do this too. Quite often a species that is not the intended target of the chemical attack is affected. The allelochemicals meant to inhibit another algae, or stop your otos from snacking, may also harm fish that wouldn't eat algae if they were starving. One species of fish in your tank may be sensitive to the chemicals, while another couldn't care less. This is one reason you want to do some large thorough water changes any time you scrape algae off the glass in your tank or use a commercial algae killer. When the algae dies, it's cell structures break down and release massive doses of allelochemicals into the water all at once. Any sensitive fish, snails, or plants may die with it.
The interactions of this chemical warfare have not been researched much yet. There are just too many species and too many variables. You tend to have a whole zoo of algae in a tank, for instance, so it's hard to narrow down what is affecting what. So you can't necessarily look at your tank and say, "gosh, if I plant crypts, and have some Anabaena algae, I'm not going to be able to keep mystery snails!"
In short, it's possible some species of algae in your tank is protecting itself from your snail by poisoning the water with something that is affecting your snail, but nothing else.
In a big ole' lake, the snail would stay on the other side of the pond. In the small closed space of an aquarium, the chemicals build up to much higher proportions, and the snail has nowhere to go.
It's also possible there's a cycling issue, temperature, disease, something ate your snail...
Wow! You read all that! Go you! You may want to read Walstad's book! As I said: dense.