Probably. Actually, irssi users seem to have problems with Unicode quite often, either because of their terminal emulator, or because of screen, which they use keep irssi online when they log off, or because of configuration problems. There basically seem to be a lot of sources for potential error with that particular combination of software.

One way of testing this would of course be to repeat the experiment with irssi and X-Chat... Most likely the same problem will appear.