Great. I figured now that you don’t understand my issue.
1) EXAMPLE ONE
Try to type in browser (you cannot stop visitors to do it, right?) https://forum.ait-pro.com as well as https://www.forum.ait-pro.com and see what you get (in both situations – P.S. – use Incognito).
2) EXAMPLE TWO
Try to type in browser (you cannot stop visitors to do it, right?) https://support.apple.com as well as https://www.support.apple.com and see what you get (in both situations – P.S. – use Incognito).
Your Forum drop out blank page (as you didn’t add support for www in DNS), what is same like my case (where I have SSL error, as I added DNS support), but substantially are same cases vs redirection.
Apple instead works as should to be. In both typing in browser, it redirect on https://support.apple.com (as you wrote, www is not ‘normal’ or usual to type, but Apple knows how MANY PEOPLE DO IT), as it should to be (what I want).
Hopefully, now you get me.