macOS proxy.pac not working?

Question

Under Network->Advanced->Proxies->Automatic Proxy Configuration I have it set to http://www.sendu.me.uk/sanger/proxy.pac, the contents of which are:

function FindProxyForURL(url, host) {
    if ((shExpMatch(host, "*sanger.ac.uk") || host == "helix.wtgc.org") && host != "wwwcache.sanger.ac.uk" && host != "wr_sb10.dnapipelines.sanger.ac.uk") {
        return "PROXY localhost:3128";
    }
    return "DIRECT";
}

I then have an SSH tunnel, set up in my ~/.ssh/config like this:

Host ssh.sanger.ac.uk
LocalForward 3128 wwwcache.sanger.ac.uk:3128

This set up works perfectly in eg. Chrome if I use the 'Proxy Helper' plugin, configured to use my proxy.pac at the above URL.

But it doesn't work in Safari. The odd thing is, it used to work for a number of years, right up until around the middle of this year. After upgrading to macOS 10.14, it still doesn't work.

Any clues as to what's wrong with it? How can I debug?