When the self-hosted runner is not connected to GitHub, it displays as ‘Offline’. This could be because the machine is offline, the self-hosted runner application is not running on the machine, or the self-hosted runner application cannot communicate with GitHub.
In your case, it seems is because the self-hosted runner application cannot communicate with GitHub. You can try to check if there are any restrictions have been set on your Mac machine to prevent the runner application from communicating with GitHub. Such as firewall or proxy.
In addition, you also can try to install a new runner on the same Mac machine to see if it can work.