Just when doing star-join, say, several inner tables are being scanned and added to server cache, and the outer table scan won't start until every inner table has been added to server cache. Then if one or more of the table scans run seriously slower than the others, when these scans finish, the other tables that finish much earlier might have expired in the cache.
However, I checked the spec for "guava cache". Seems that cache expiration and cleanup does not always actively happen, so that the cache map won't be locked in conflict with user writes. So I suppose warning here can act as a good reminder for potential issue whether or not the problem actually occurs.