I gave up trying to get that working on my setup – I never had any luck. Everything looked good in the gui but the forwards never went out externally. Going into SoGo and forwarding from within worked perfectly and is a 30 second “fix” instead of fiddling around with rules and such. Sure, you need to have SoGo installed but meh - at least it’s working.
I seem to recall that it had to do with either sieve or postfix not using the “sender-dependent transport” for sending the forwards properly. I remember seeing in the logs it defaulted those external outbound email forwards to port 25, which is blocked by my ISP and my sender transport is configured to send over port 2525.