`C-x C-;` freezes emacs

Question

This hasn't been happening every time, though once it starts it's consistent while emacs is running at least. It claims to be bound to comment-line as usual, but I can't actually run it through C-h k because it freezes emacs there too.

I have ivy or helm or something installed so I when I just type C-x it pops up a buffer full of the keystroke options on the bottom of the frame. When I type C-x C-;, it doesn't respond to the C-;, but the menu still opens normally a few seconds after. However, before or after, emacs doesn't respond to any keystroke.

Once this happens, emacs becomes completely unresponsive. The only ways out of this state are focusing on a different window (I'm on Ubuntu) and then refocusing on emacs frame; and pressing ESC. Notably, C-g does nothing. Both of these leave emacs still on C-x and waiting for the next keystroke, which works normally -- except of course that C-; repeats the problem from the start.

I thought it might be my OS hijacking C-; or something, but I couldn't replicate it in anything other than emacs. I'm running emacsclient against a local daemon. I had spacemacs, then moved to prelude, and this has been consistent in both. I'm on the latest Ubuntu, with the emacs 27 which is the latest in the apt repos.

Edit: this also happens with C-; by itself, i.e. not after C-x.