Oh nice catch! This was a miss from me DRYing up the local server start code into a helper function. I copied the helper from places that used 127.0.0.1 and missed that port forwarding specifically didn't have it. It's fine to remove it again.

As far as I know, the privilege port restriction should apply globally regardless which address you bind it with, or at least all non-local addresses (based on my experience)
I don't know exactly why one syntax would work but not the other.
Presumably it has something to do with the OS specifically.

I simply compared the source between the last known working version and the one stopped working, then tested the 2 syntax locally in one single main.go file and confirmed that's the cause.

@cmbrose @jasonw-rr Either of you have time to prepare PR for this?

Wow, thanks for the prompt response and turnaround!