Use dx_max_sort_old and not dx_max_sort when deciding what direction to sort in the recursion.

src/runner.c

@@ -702,7 +702,7 @@ void runner_do_sort(struct runner *r, struct cell *c, int flags, int cleanup,
       if (c->progeny[k] != NULL && c->progeny[k]->hydro.count > 0) {
         /* Only propagate cleanup if the progeny is stale. */
         runner_do_sort(r, c->progeny[k], flags,
-                       cleanup && (c->progeny[k]->hydro.dx_max_sort >
+                       cleanup && (c->progeny[k]->hydro.dx_max_sort_old >
                                    space_maxreldx * c->progeny[k]->dmin),
                        0);
         dx_max_sort = max(dx_max_sort, c->progeny[k]->hydro.dx_max_sort);