How do I keep browsers from adding www prefix to my subdomain?

I uploaded beta.mysite.com to my web server. When I test the site on my server, I can browse beta.mysite.com fine without issue. For some reason on my local machine, no matter what browser I use, Chrome, Safari or Edge, it insists on redirecting beta.mysite.com to www.beta.mysite.com, resulting in an error page. Even if I remove the www prefix, it puts it back on.

Since it doesn't happen on my server, I presume it is entirely browser related. This behavior is incredibly frustrating and I can't see why it would be implemented at the browser level. Moreover, the www convention is rarely used on another subdomain address.