It’s a bug, and actually caused by IFD: nix flake check breaks on IFD in multi-platform flake · Issue #4265 · NixOS/nix · GitHub
nix flake show works perfectly fine if you have packages for multiple systems, but you cannot use IFD in your flakes at the moment.
Arguably you shouldn’t use IFD anyway, I think flakes need some way to allow auxilliary commands to do repository setup that hook into nix flake update so we can avoid needing IFD in the first place.