Tmux session disconnect does not preserve session content when reconnecting

I need to copy a 200gb+ file to multiple servers from one server. I have a sftp transfer running in multiple tmux sessions from the one server.

When I press ctrl+b, d to exit the tmux instance and rejoin with tmux -L , it opens the session to a new console session and does not preserve the progress report of the running sftp.

Is there a way to prevent this behavior? I am using Tmux 2.7.

Commands:

tmux new-session -s Upload1tmux -L Upload1sftp user@ipcd /path/to/upload/location    put filectrl+b,dtmux -L Upload1

The sftp process still runs in the background, as I can see the file size increasing every minute.