This site doesn't provide a 404 error after i try being able to access an invalid page URL. This site is really a subdomain, I own both primary, and also the subdomain

The issue is only happening using the subdomain where, basically enter something similar to, still it shows the home page with damaged image links rather than giving a 404 ERROR.

There's likely a guide spinning demands for missing content towards the primary page. Look at your logs for 302 or 301 redirects.

Some sites make use of a custom error page. Otherwise coded well, they'll return a 200 status as opposed to the status that triggered it.