Did that help? Nothing else pops into my head about why it wouldn't work, but the fact that the only thing you did to make it break was add an interface doesn't really help my hypothesis.

If that's not it, you could try a network snoop and see what packets happen. The router might be getting confused and sending it out of the wrong interface for some reason, or maybe it doesn't want to generate traffic from external addresses pointed to the internal interface, despite that being a perfectly valid thing to do.