Okay this is getting stranger. I did - I believe - what you said and then my browser started giving me errors that the connection was being reset and to try again when I tested it. So I went back to the server behavior to double check, and restricting content was there, but not for a space. Something about begins with.
I deleted it and took screen shots so you can see. In the first two capture jpgs you see me make the validation. Then I saved the file, came back in in the third one you can see it's not what I saved. I've included the file again too.
I'm not sure what's going on. I hope it's not something stupid I'm doing.