Jon - It appears that losing the appendage is normal for the Stomatella varia.
I found this info in a article by Shimek:
Stomatella varia (Figure 11)
Almost slug-like, but possessing an auriform, cap-like shell, individuals of Stomatella seem unlikely to be Trochoideans. However odd their appearance, their internal anatomy puts them squarely in this group. Stomatella individuals may reach lengths of about an inch and a quarter (3 cm), and as their specific name of "varia" implies, they may be almost any color, although gray, black, and mottled colors tend to predominate. They are quite good grazers, and also reproduce well in aquaria lacking hermit crabs and shrimp predators. Their shell shape precludes their retraction into the shell, but evolution has given them a rather neat way to escape many predators. The rear portion of their foot can break off, much like some lizards' tails, when they are threatened by a predator. The detached portion then writhes, and twists and otherwise distracts the predator while the snail, presumably, makes good its escape. Additionally, they also tend to be nocturnal and avoid a lot of visually-oriented predators in this way.
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Figure 11. A Stomatella varia about half an inch long. This individual's foot is retracted and the shell
is quite evident. The foot commonly extends far behind the shell and, additionally, often obscures
part of the shell. The odd shell shape, for a Trochoidean, is evident.
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