Less than a week ago I complained about nearly the exact same issue and received the typical “nixy” response “that’s what the cache is for”. Well, that’s excellent news, when the cache actually works as expected. I see this a lot with my flake.nix per repo configurations too. The flake.nix never changes, yet it inexplicably rebuilds packages sometimes
I guess our system's nixpkgs moves forward while the nixpkgs in our project flake.nix stays the same. So eventually the nixpkgs of the project is garbage collected, because it isn't pinned by the system, and it needs to redownload and rebuild?
I don't know.
I try to `nix flake update` my system configuration every 2-3 months.
Arguably, running Arch and not updating frequently is a pain, too.