<C-CR> not working under tmux

I use neovim in tmux for development and have set up <C-CR> as a keymap for cmp completion. It was working well until just a while ago and I don't know what happened.

<C-CR> seems to register as a simple <CR> when I'm under tmux and so doesn't cmp complete in neovim. It's not a problem with cmp as <C-y> is also bound to that and it does work. It also isn't a problem with neovim or the terminal emulator itself, as I tried this on foot, alacritty, kitty and gnome-terminal. In all cases, <C-CR> works when not in tmux, but when tmux it doesn't work. It was working fine just yesterday, and I don't know why it isn't now or how to fix it.

Has anyone had this kind of issue or knows how to fix it?

EDIT: Found a fix, though I don't know why it was working earlier and stopped working suddenly
https://github.com/tmux/tmux/wiki/Modifier-Keys#extended-keys
In tmux.conf, add

set -s extended-keys on
set -as terminal-features 'xterm*:extkeys'