How recent is your experience with Mikrotik? How old is the hardware you tried out?

This. Also, make sure the proxy is proxying websocket traffic as well. I do it with pure nginx like this:

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://192.168.1.100:8123/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

        # WebSocket support
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }
}