Default owner_xchan to $observer (sender) in Activity::store() - this is because in case where an announce holds a relayed activity we drop the announce and process the relayed activity only. In that case actor.id as set in Activity::decode_note() will not be the correct owner. In other cases actor.id and sender should be identical.

Zotlabs/Lib/Activity.php

@@ -2852,6 +2852,7 @@ class Activity {
 		}
 
 		$allowed = false;
+		$item['owner_xchan'] = $observer_hash;
 
 		// TODO: not implemented
 		// $permit_mentions = intval(PConfig::Get($channel['channel_id'], 'system','permit_all_mentions') && i_am_mentioned($channel,$item));