fix inherited backend var deps

[benedikt-bartscher]

Closes #2714

[masenf]

Interesting, i wonder why it wasn't getting marked dirty in the parent state and being pushed down in the substates via this block

        if (
            types.is_backend_variable(name, self.__class__)
            and name not in RESERVED_BACKEND_VAR_NAMES
        ):
            self._backend_vars.__setitem__(name, value)
            self.dirty_vars.add(name)
            self._mark_dirty()
            return