source/blender/blenkernel/intern/paint.cc

@@ -2266,6 +2266,8 @@ PBVH *BKE_sculpt_object_pbvh_ensure(Depsgraph *depsgraph, Object *ob)
     return pbvh;
   }
 
+  ob->sculpt->islands_valid = false;
+
   if (ob->sculpt->bm != nullptr) {
     /* Sculpting on a BMesh (dynamic-topology) gets a special PBVH. */
     pbvh = build_pbvh_for_dynamic_topology(ob);

source/blender/editors/sculpt_paint/sculpt_ops.cc

@@ -229,6 +229,8 @@ static int sculpt_symmetrize_exec(bContext *C, wmOperator *op)
       return OPERATOR_CANCELLED;
   }
 
+  SCULPT_topology_islands_invalidate(ss);
+
   /* Redraw. */
   SCULPT_pbvh_clear(ob);
   WM_event_add_notifier(C, NC_OBJECT | ND_DRAW, ob);