When doing URL forwarding from my custom domain from registrar to GitHub, mobile view doesn't work and gives tablet view instead

Question

I have a website set up so that my custom domain forwards to my GitHub pages website. I set this so that it's easier to host static websites.

When I view my website through custom domain on mobile view, it gives me the tablet view instead. When I visit using the github.io link on mobile view, it works perfectly fine, and mobile view is as it's supposed to be.

My registrar is register.ca.