Brain damage? There is no way to suggest a brain damage on a snail. There is no such record for this with snails. There is no reason why your snail would go dormant just like that if your water conditions would be fine but you do need to address your ammonia. Some snails can be sensitive to ammonia.
Just for reference, your water is soft and acidic. Nerites need plenty of calcium as they absorb a lot of calcium for their shells. Acidic water can only erode the shells in the long run. I had someone in a local forum complained his nerites suffered shell erosion. It turns out his water is soft and acidic and never bothered to dose calcium at all.
I have had the snails for a little over a month.
They where in my 29 gallon tank for about 2 weeks or so, ammonia was in the tank for half or less then half of that time.
They have been in the clean fresh shrimp only tank now for about 2 weeks.
Ammonia can be used to kill terrestrial snails and slugs. If aquatic snails react similarly to ammonia it wouldn't matter how long it was present. The fact that it was even exposed to ammonia could have caused damage.