Well, it is definitely broken in 13.1, and I don't think there is anything you can do to make it work.
So your workarounds are:
Use /SCRIPT instead of /EXEC
Download the 13.2 beta and use that instead.
If you can use the beta, please do. The nice thing about that is if you find a problem, we can usually have a fix for you in a day or two. I'd really like to see a lot more beta users. It's good for you, and good for Toad.