Thanks for the report! I took a look, and this appears to be a pnpm version mismatch in the Nix package rather than a problem with the dependency hash.
Discourse v2026.1.4 declares pnpm@10.28.0, while the Nix expression uses the moving pnpm_10 package, which currently resolves to 10.34.4. That newer pnpm version rejects some dependency aliases used by this Discourse release.
The likely Nixpkgs fix is to pin the derivation to an older compatible pnpm, possibly the existing pnpm_10_29_2 package, or exactly 10.28.0 and then update pnpmDeps.hash if necessary.
Since the Nix package is community-maintained, that fix would need to be made in Nixpkgs, but hopefully this points the maintainers in the right direction.