Today I finally switched my long-running IRC screen session to tmux. I rarely disconnect so I was simply waiting for a "good" time.
After a few minutes I noticed that my irssi scrollback was misbehaving; only half of my window would redraw when I pressed PgUp of PgDn.
I had manually set "TERM=xterm" in my bashrc; however, tmux advises only using "screen" or "screen-256color" as the term. I have no idea why I had specified term within my bashrc; removing the line and restarting tmux corrected the issue.