Face set visibility is already flushed to all mesh elements in the right
way in SCULPT_visibility_sync_all_face_sets_to_vertices, calling
BKE_mesh_flush_hidden_from_verts is legacy code from the previous
visibility system that was causing the vertex visibility to take
priority over face sets. When hidding a single loop, all vertices of
those faces are still visibile, so this line was tagging all loop faces
visible. This was leaving mesh/face set visibiltiy in an unconsistent
state.