Nvm, I only have the problem when I connect to a 14.04 machine, then start byobu, then connect to a 18.04 machine, and then start neovim. So the correct (14.04) versions of tmux and byoubu are
tmux: 1.8.5
byobu: 5.77-0ubuntu1.2
If I connect to the 18.04 machine directly, then start byobu + neovim (with the versions as in my previous comment), no problems, so I guess this issue is fixed!
Nvm, I only have the problem when I connect to a 14.04 machine, then start byobu, then connect to a 18.04 machine, and then start neovim. So the correct (14.04) versions of tmux and byoubu are
tmux: 1.8.5
byobu: 5.77-0ubuntu1.2
If I connect to the 18.04 machine directly, then start byobu + neovim (with the versions as in my previous comment), no problems, so I guess this issue is fixed!