\[SOLVED\]
I'm not sure what's happening, but when using Emacs inside Tmux, I can't type an asterisk? Instead, the minibuffer says "M-O j is undefined". Anyone know what this actually means?
Edit: I'm using uxrvt, but the same happens in xterm.
Edit 2: Ok, at least I have a work around - holding shift and it works??!
Edit 3: Turns out it was urxvt that was breaking. Switched to Alacritty and everything worked