I have kept aquarium snails of different kinds but never encountered such an issue. I will still try to tell you as much as possible, in case anything can help.
Water quality. This is the most easy answer. An excess of pollution could be burning up these sensitive parts. No matter what the issue is, keeping your water extra clean is your best bet, moreso until you know the exact problem and cause. I assume you know how to manage a fishtank and are aware of the nitrogen cycle.
Temperature of the water could be wrong. You can search online what temperature each snail prefers.
A parasite, fungus or disease. This seems very likely in your case, it looks like an epidemic. Maybe someone can recognize these symptoms and identify the issue. Then a medication could be recommended. I would suggest you bring one specimen or more to a specialist of snails if you can find any. Water quality will also help the healing.
A toxic chemical in the water. Did you add any chemical product recently to your water? New piece of decoration, new piece of filtration, a heater, new substrate, plants, anything? Think if you added something when this problem started. Snails are fragile when it comes to copper for exemple. It could be a paint on a piece of decoration.
Physical injury like you mentionned, fish attacks (you don't have fishes), or an uncovered filter intake or something, even some unwelcome invertebrate could be attacking them, but since it's spreading among your snails, this is probably not the cause.
Diet as Trond mentionned. Maybe they are missing something, either calcium or some vitamin. Besides sepia bone and vegetables, you could also give them actual fish food.
What you can do:
Keep the water clean, find a snail specialist or vet or hope someone here can diagnose what is happening.
You can try to separate healthy and not-healthy specimens with a second tank and see what happens. It might help you discover the root of the problem. You can restart a healthy population even with a few healthy individuals in a different tank. Be sure not to mix in water from the not-healthy tank, start it completely from scratch or from a healthy tank. Start as soon as possible to isolate healthy specimens before they catch it too. You can even put them in quarantine in glasses of water.
It might be better to focus on restarting a healthy colony rather than try to heal all the current snails, considering their reproduction rate.
Snails might easily regrow healthy tentacles once the issue is resolved.
I like your concept of a snail-only tank. Wish I could help more, very original question. Good luck.