I'm glad that it now works for you, but I'm not sure to understand what was the issue. I'd like to make sure that the current behavior is the right one, if it's different from the old behavior. Your problem was caused by a rewrite_metadata with a rewriting of the form ("title","..$(title)..")?
As far as I understand the code of rewrite_metadata, it cannot currently add missing metadata but only change existing metadata. I may be wrong, but if that's the case then I think that we should change this behavior. To me, the metadata resulting from a rewrite should either: (1) have all fields of both the rewriting rules and the input metadata or (2) have only the fields of the rewriting rules -- cause the others can't be controlled that well.