Domain Name Forwarding not working with github pages

I had my blog on github pages with a custom domain name blog.tamizhvendan.in and today I've moved it to a new domain name www.p3programmer.com by updating the CNAME record with www.p3programmer.com.
The new domain name is working fine.

As it is a breaking change, I would like to have a domain redirection which redirects the old one to the new one. I've configured the same in my DNS Server (BigRock) using their Domain Forwarding System. After that, tamizhvendan.in is redirecting correctly to www.p3programmer.com but blog.tamizhvendan.in resulting in 404 in github pages. By BigRock's documentation, for subdomains will be redirect to path www.example.com\{subdomain} i.e in this scenario it should redirect to www.p3programmer.com\blog but it is not redirecting.

I've added a CNAME record in the tamizhvendan.in account with the value pointing to www.p3programmer.com but yet I am getting github page's 404 page error.

Here is my dig output of blog.tamizhvendan.in

; <<>> DiG 9.10.1-P2 <<>> blog.tamizhvendan.in +nostats +nocomments +nocmd
;; global options: +cmd
;blog.tamizhvendan.in.          IN      A
blog.tamizhvendan.in.   2658    IN      CNAME   www.p3programmer.com.
www.p3programmer.com.   1818    IN      CNAME   tamizhvendan.github.io.
tamizhvendan.github.io. 427     IN      CNAME   github.map.fastly.net.
github.map.fastly.net.  30      IN      A       23.235.44.133

Dig output of www.p3programmer.com

;; global options: +cmd
;www.p3programmer.com.          IN      A
www.p3programmer.com.   1740    IN      CNAME   tamizhvendan.github.io.
tamizhvendan.github.io. 349     IN      CNAME   github.map.fastly.net.
github.map.fastly.net.  30      IN      A       23.235.40.133