When I try and change the Denied Message it has two areas, message and body. So if the message is “Access Denied” and the body is “Sign up for a product to access this” it’s fine. The issue is you can only change one. The message or the body, not both.
So if you wanted to change it to:
Message: Members Only
Body: Sign Up for FREE membership today with validated email.
Whichever field you last updated will change when you hit save. The other will just go back to whatever is was. No matter what combination you do, you always lose the other field when you save.