I have same in my 29g. I have a spraybar near the surface and I often find a bunch of small MTS floating on the surface. But they're all alive and well, and eventually, they make it to the bottom. As to why it happens, I have no clue.
I also noticed this recently on a 7g shrimp tank. I had fed some freeze-dried shrimp to them recently, which also did a lot of floating before it was waterlogged. I just supposed that the snails were attracted to the proteins on the surface from the food. Just a wild guess, really.
I've kept MTS from 3 different sources for a year and never had a floater.
my guess is there might be some strong water flow against the glass,
so when they come up at night for air, they are getting swept away.
I always have a floating Ramshorn or two, but that's because they
keep adjusting their buoyancy so they can eat algae on finer leaves.