  We investigate the effect of two-body loss due to chemical reactions on
quantum magnetism of fermionic polar molecules in an optical lattice. We show
that an interplay between dissipation and strong long-range interactions leads
to formation of metastable ferromagnetic clusters. The spin states of clusters
are controlled by interaction parameters and reflect the symmetry of
interactions. The size of clusters strongly depends on the initial
configuration of molecules due to Hilbert-space fragmentation during
dissipative many-body dynamics. We construct an effective model to show the
emergence of metastable states as quasi-dark states. Application to quantum
simulation of the spin-$S$ Heisenberg model is discussed.
