Well it does looks like starting a new campaign after you finished one erase the previous one and all your time record... it's a pretty retarded thing...
One way I saw to have multiple campaigns at the same time is to keep one save from your previous campaign.

This way by reloading it you can access the previous campaign.

Pretty bad system from Blizz really in my opinion, DoW2 system would be so much better but I guess it was made this way to avoid multiple people sharing an account to play SP...