We coincidentally had an installer workgroup meeting this morning and I asked about this. The developer on the detsys installer thinks this sounds like some reports they have about trouble caused by uninstalling Nix without uninstalling nix-darwin first, and having some of nix-darwin’s artifacts/changes dangling.
https://github.com/DeterminateSystems/nix-installer/issues/528
If that sounds like it might describe your case, you may need to manually uninstall nix-darwin. Judging from Uninstall nix-darwin manually · Issue #542 · LnL7/nix-darwin · GitHub, it sounds like there’s heavy emphasis on the manually bit. (I haven’t looked around further to see if there is a clear set of manual instructions out there…)