Last modified: 2013-07-31 16:52:19 UTC
Searching for "talk:" searches for the word talk in the namespace talk. It should instead search for all articles in the talk namespace.
Surprisingly, this seems like expected behaviour. It looks like it has been in place since 2010 at least. I'm not really sure I can consider it a bug given that someone thought it was a good idea at the time. I don't think it is right, but I don't think it is wrong enough to fix an intentional change.