Index: trunk/src/org/openstreetmap/josm/data/osm/Relation.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/Relation.java	(revision 11315)
+++ trunk/src/org/openstreetmap/josm/data/osm/Relation.java	(revision 11316)
@@ -451,6 +451,10 @@
         addToBBox(box, new HashSet<PrimitiveId>());
         if (getDataSet() != null)
-            bbox = box; // set cache
+            setBBox(box); // set cache
         return new BBox(box);
+    }
+
+    private void setBBox(BBox bbox) {
+        this.bbox = bbox;
     }
 
@@ -465,5 +469,6 @@
     @Override
     public void updatePosition() {
-        bbox = getBBox();
+        setBBox(null); // make sure that it is recalculated
+        setBBox(getBBox());
     }
 
@@ -472,5 +477,5 @@
         super.setDataset(dataSet);
         checkMembers();
-        bbox = null; // bbox might have changed if relation was in ds, was removed, modified, added back to dataset
+        setBBox(null); // bbox might have changed if relation was in ds, was removed, modified, added back to dataset
     }