(Please Note: I cross posted this on my Club Penguin Bugs website)
Back when the Music Jam 2011 Party was released on Club Penguin I posted a guide on the whole party, talking about everything important such as free items, the member rooms, special catalogs, etc. At the very bottom I said the Explorer stamp is unlockable, because it should be. However, I soon received a few comments from penguins saying it was not working.
I emailed Club Penguin but did not get a response. Due to this I emailed them the other day for a second time, and this time I got a reply. This is what they said:
You’re probably squinting to read that, so let me tell you what Club Penguin said. The CP support representative said the Club Penguin Tech Team is aware of the bug of the stamp not being unlockable. They are working hard to fix it, but do not know when it will be unlockable again. Either way, hopefully it is soon so penguins can get it before the party ends on July 4th!