  The Fermat--Steiner problem is to find all points of the metric space Y such
that the sum of the distances from each of them to points from some fixed
finite subset A = {A_1, ..., A_n} of the space Y is minimal. This problem is
considered in the case when Y=H(X) is the space of non-empty compact subsets of
a finite-dimensional normed space X endowed with the Hausdorff metric, i.e.
H(X) is a hyperspace over X. The set A is called boundary, all A_i are called
boundary sets, and the compact sets that realize the minimum of the sum of
distances to A_i are called Steiner compacts. In this paper, we study the
question of stability in the Fermat--Steiner problem when passing from a
boundary consisting of finite compact sets to a boundary consisting of their
convex hulls. By stability here we mean that the minimum of the sum of
distances does not change when passing to convex hulls of boundary compact
sets. The paper continued the study of geometric objects, namely, hook sets
that arise in the Fermat--Steiner problem. The results obtained revealed some
geometry of the relationship between Steiner compacts and boundary sets. On its
basis, a sufficient condition for the instability of the boundary in H(X) was
derived.
