Math commit in stanheaders does not exist

Summary:

When running

git clone --recurse-submodules git@github.com:stan-dev/rstan.git

I'm getting

fatal: remote error: upload-pack: not our ref 520cdec64c004d19d64b01edafe44379977d1a5e
fatal: the remote end hung up unexpectedly
Fetched in submodule path 'StanHeaders/inst/include/mathlib', but it did not contain 520cdec64c004d19d64b01edafe44379977d1a5e. Direct fetching of that commit failed.

Also going to the mathlib link here does not go anywhere

Description:

Full output: cloning from the rstan repo

git clone --recurse-submodules git@github.com:stan-dev/rstan.git
Cloning into 'rstan'...
remote: Enumerating objects: 212, done.
remote: Counting objects: 100% (212/212), done.
remote: Compressing objects: 100% (125/125), done.
remote: Total 40339 (delta 137), reused 133 (delta 86), pack-reused 40127
Receiving objects: 100% (40339/40339), 171.52 MiB | 11.07 MiB/s, done.
Resolving deltas: 100% (19471/19471), done.
Submodule 'StanHeaders/inst/include/mathlib' (https://github.com/stan-dev/math.git) registered for path 'StanHeaders/inst/include/mathlib'
Submodule 'stan' (https://github.com/stan-dev/stan.git) registered for path 'StanHeaders/inst/include/upstream'
Cloning into '/home/steve/stan/origin/rstan/StanHeaders/inst/include/mathlib'...
remote: Enumerating objects: 34, done.        
remote: Counting objects: 100% (34/34), done.        
remote: Compressing objects: 100% (21/21), done.        
remote: Total 499024 (delta 16), reused 27 (delta 13), pack-reused 498990        
Receiving objects: 100% (499024/499024), 454.77 MiB | 5.05 MiB/s, done.
Resolving deltas: 100% (357710/357710), done.
Cloning into '/home/steve/stan/origin/rstan/StanHeaders/inst/include/upstream'...
remote: Enumerating objects: 49, done.        
remote: Counting objects: 100% (49/49), done.        
remote: Compressing objects: 100% (40/40), done.        
remote: Total 233553 (delta 17), reused 25 (delta 8), pack-reused 233504        
Receiving objects: 100% (233553/233553), 296.19 MiB | 5.70 MiB/s, done.
Resolving deltas: 100% (164657/164657), done.
fatal: remote error: upload-pack: not our ref 520cdec64c004d19d64b01edafe44379977d1a5e
fatal: the remote end hung up unexpectedly
Fetched in submodule path 'StanHeaders/inst/include/mathlib', but it did not contain 520cdec64c004d19d64b01edafe44379977d1a5e. Direct fetching of that commit failed.

Current Output:

going into the repo and running git status yields

On branch develop
Your branch is up to date with 'origin/develop'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
  (commit or discard the untracked or modified content in submodules)
	modified:   StanHeaders/inst/include/mathlib (new commits, modified content)
	modified:   StanHeaders/inst/include/upstream (new commits)

no changes added to commit (use "git add" and/or "git commit -a")

Expected Output:

I would expect it to have the pinned versions and not new things

RStan Version:

74f847ca1f61944b5dccc173f3eae435a7b6f1cb

R Version:

The version of R you are running (e.g., from R.version.string)

Operating System:

Ubuntu 20.02

$ git --version
git version 2.25.1

Start with the RStan commit 74f847ca1f61944b5dccc173f3eae435a7b6f1cb and inspect the submodule references for StanHeaders/inst/include/mathlib and StanHeaders/inst/include/upstream. Reproduce the recursive clone and verify that the referenced mathlib commit is available; done means the clone succeeds with the pinned submodule versions and no modified submodule state.