I've had hermits of 2 kinds in my tank, and I've not seen them ever attack astrea snails. Astreas are actually one of the only kind my crabs have not killed. I have about 10 blue legged hermits and a scarlet hermit. They did like the Llynassa snails when I had them (do not reccomend them to anyone now) and the turbos.
I don't think the crabs are your problem, just that you keep losing them from tipping over.
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