This is an accepted solution.
Your "www.mystore.com" URL is a subdomain of your root (mystore.com), so it should typically be listed under your available domains in your admin. Any listed domains there will redirect to whichever one is listed as your primary domain, so you might be able to fix this by making the "www." domain your primary. To do this, navigate to Online Store > Domains > Change primary domain > select the one starting with "www.". Let me know if that works! If not, I can dig a little deeper for you to see if this may not be possible. Cheers.