I think I was somehow bitten by Default Nix installations don't include user channels in NIX_PATH, causing inconsistent tool behavior · Issue #2033 · NixOS/nix · GitHub.
I had realised that my macOS installs were a bit outdated compared to NixOS. So I updated the channels with
sudo -i
nix-channel --update
I think this issue might have been caused by me mismatching home-manager/nixpkgs 20.09/unstable… When changing my channels after running into https://github.com/nix-community/home-manager/issues/1645 didn’t work, that’s when I realised that my local channels were still out of date.