http://tickets.musicbrainz.org/browse/BB-78?focusedCommentId=36572&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_36572

I've done some testing, and it looks like this occurs when the entities are swapped after selecting a second entity, but before the relationship has rendered. My current theory is that the returned entities from the server are overwriting the user's swapped entities, which is somehow causing an entity to be removed.