  Heavy quarkonium at finite temperature has been the subject of intense
theoretical studies, for it provides a potentially clean probe of the
quark-gluon plasma. Recent studies have made use of effective field theories to
exploit in a systematic manner the hierarchy of energy scales that characterize
the system. In the case of a quarkonium in a medium whose temperature is
smaller than the typical momentum transfer in the bound state but larger than
its energy, the suitable effective field theory is pNRQCD_HTL, where degrees of
freedom with energy or momentum larger than the binding energy have been
integrated out. Thermal effects are expected to break Poincar\'e invariance,
which, at zero temperature, manifests itself in a set of exact relations
between the matching coefficients of the effective field theory. In the paper,
we evaluate the leading-order thermal corrections to the spin-orbit potentials
of pNRQCD_HTL and show that Poincar\'e invariance is indeed violated.
