Download seems to expect a user agent string. Updating the label to include a user agent string worked. Snippet:

openvpnconnectv3)
# credit: @lotnix
name="OpenVPN Connect"
type="pkgInDmg"
if [[ $(arch) == "arm64" ]]; then
pkgName="OpenVPN_Connect_[0-9_()]arm64_Installer_signed.pkg"
elif [[ $(arch) == "i386" ]]; then
pkgName="OpenVPN_Connect[0-9_()]_x86_64_Installer_signed.pkg"
fi
downloadURL="https://openvpn.net/downloads/openvpn-connect-v3-macos.dmg"
expectedTeamID="ACV7L3WCD8"
curlOptions=( -H "User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/109.0.0.0 Safari/537.36" )
;;