This is happening on v7.4.
> I don't see this.
Hmm. This seems to be an issue with zsh and or tmux. Executing the command which vim does also hangs.
$ /bin/zsh -c "(tmux load-buffer -) < /tmp/file"
[warn] Epoll ADD(1) on fd 0 failed. Old events were 0; read change was 1 (add); write change was 0 (none): Operation not permitted
^C^C^C^C^Czsh: killed /bin/zsh -c "(tmux load-buffer -) < /tmp/file"