nat: try known mapping peers before observer-only fallback

src/routing_layer/nat_detection.c

@@ -290,7 +290,7 @@ static int select_pair(struct nat_socket *state) {
             }
         }
     }
-    if (!best.group && single.group && !state->using_fallback) {
+    if (!best.group && single.group && !state->using_fallback && !state->prefer_fresh) {
         best = single; observer_address = single_address; memcpy(observer_key, single_key, 32);
     }
     if (!best.group) {