For people who aren't familiar with nix-serve-ng, it's basically a much more efficient alternative to nix-serve that I open sourced from my previous job.
The PR to fix this is up here:
Big thanks to Robert Hensing for helping me fix this
This is really nice since the whole point of nix-serve-ng is to be a strictly better drop-in replacement for nix-serve, but this wasn't the case on macOS (where nix-serve would work and nix-serve-ng didn't work up until now).