Mystery Snail Mystery death

I got all 5 of my mystery snails when they were about half their current size. They all grew very big very quick! About to ping pong ball size in 2-3 months.

In the past month they've started dying spontaneously one at a time. I had my third death today and I've got 2 snails left. I'm wondering if this is just their lifespan or if this is unusual. I've had them for about 5 months.