Wow, I thought I was pretty savvy about the differences between Manifest versions; however, I had no clue about the CNAME part. Any idea what specifically became unsupported in v3 that caused this?
Without the webRequest API, it is unable to intercept raw web requests on the fly or query live DNS/CNAME records or dynamically reroute matching rules. The MV3 version is basically just a static list of rules that is only updated when you update the extension itself