I have the exact same problem as this but I don't understand the solution.
I have 2 different domain name(with different hosting/provider) domain-one.com and domain-two.com which I want sub.domain-one.com to point to sub.domain-two.com using CNAME. All have been setup and propagate correctly but now when I go to sub.domain-one.com it goes to basic page of the server. If I go directly to sub.domain-two.com the page load correctly.
I've tried to add the subdomain using Addon Domains on cPanel but the problem persist.