Forward with URL masking is breaking the css for a site built with react gatsby and hosted on netlify

Question

I have a site built on Gatsby JS (React) and whenever I forward a subdomain with masking the css for mobile breaks. My domain registrar is godaddy and forward without masking works

For example:

abc.mydomain.com -> mysite.netlify.com (No Masking) ->Responsive
abc.mydomain.com -> mysite.netlify.com (Masking) -> No responsiveness

Any help is appreciated!