  We consider the stability of fragile topological bands protected by
space-time inversion symmetry in the presence of strong electron-electron
interactions. At the single-particle level, the topological nature of the bands
prevents the opening of a gap between them. In contrast, we show that when the
fragile bands are half filled, interactions can open a gap in the many-body
spectrum without breaking any symmetry or mixing degrees of freedom from remote
bands. Furthermore, the resulting ground state is not topologically ordered.
Thus, a fragile topological band structure does not present an obstruction to
forming a "featureless insulator" ground state. Our construction relies on the
formation of fermionic bound states of two electrons and one hole, known as
"trions". The trions form a band whose coupling to the electronic band enables
the gap opening. This result may be relevant to the gapped state indicated by
recent experiments in magic angle twisted bilayer graphene at charge
neutrality.
