Nix develop fails due to missing godot-haskell revision

When trying to enter the Simula VR dev shell on the master branch, using nix develop it fails with this error message:

warning: ignoring untrusted flake configuration setting 'extra-substituters', pass '--accept-flake-config' to trust it (may allow the flake to gain root, see the nix.conf manual page)
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys', pass '--accept-flake-config' to trust it (may allow the flake to gain root, see the nix.conf manual page)
error:
       … while evaluating an expression to select 'drvPath' on it
         at «internal»:1:552:
       … while evaluating strict
         at «internal»:1:552:
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Cannot find Git revision 'b06876dcd2add327778aea03ba81751a60849cc8' in ref 'refs/heads/master' of repository 'https://github.com/SimulaVR/godot-haskell'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.

I haven't tested any other branches yet. I'll assume that extra-substitures shouldn't make a difference for finding the missing rev.
I have cloned the repo, including it's submodules, with the recursive flag as stated in the Readme.

Start with the README instructions for entering the Simula VR development shell and reproduce the failure with nix develop on the master branch. Inspect the dependency reference for https://github.com/SimulaVR/godot-haskell and the missing revision b06876dcd2add327778aea03ba81751a60849cc8; done means the development shell can be entered successfully.