Checking out a branch that contains a new submodule reference

If I clone a repository with git --clone-recursive to checkout the repo and all referenced submodules and then attempt to checkout a branch that contains a new submodule reference, an error prevents me from checking that branch out.

Initial configuration

Create a new main module repo at https://gitlab.com/<company>/<project>/mainmodule with a README.md at gitlab.com.

Create a new child submodule repo at https://gitlab.com/<company>/<project>/childsubmodule with a README.md at gitlab.com.

In the childsubmodule project, Settings -> CI/CD -> Job token permissions add <company>/<project>/mainmodule entry so that mainmodule can access childsubmodule in CI pipelines.

Clone https://gitlab.com/<company>/<project>/mainmodule to a new local folder using:

git clone --recurse-submodules git@gitlab.com:<company>/<project>/mainmodule.git

Create a new branch:

cd mainmodule
git checkout -b demo/add_a_submodule

Add a relative path submodule reference using:

git submodule add ../childsubmodule

Add the changes (the .gitmodules and new submodule folder) and commit and push the new branch to the remote:

git add -u
git commit -m "Add a submodule to branch"
git push -u origin demo/add_a_submodule

Failure Case: Cloning mainmodule recursively getting submodules

Clone https://gitlab.com/<company>/<project>/mainmodule to another new local folder using:

git clone --recurse-submodules git@gitlab.com:<company>/<project>/mainmodule.git test

Try and checkout the branch:

cd test
git checkout demo/add_a_submodule

Failure: The .gitmodules file is updated but the branch doesn't change and these errors are shown:

  fatal: not a git repository: ../.git/modules/childsubmodule
  fatal: could not reset submodule index

Success Case: Cloning mainmodule and then getting submodules explicitly

Clone https://gitlab.com/<company>/<project>/mainmodule to another new local folder without getting submodules using:

git clone git@gitlab.com:<company>/<project>/mainmodule.git test2

Before changing branch, get the submodules with:

cd test2
git submodule update --init --recursive

Try and checkout the branch:

git checkout demo/add_a_submodule

Update the submodule:

git submodule update --init --recursive

Success: The branch switch is successful and the new submodule is added / updated.

I have repeated the steps using a local Gitea server and in the failure case I do not see the same failure to checkout the branch. Instead the checkout succeeds and then I'm able to update the newly added submodule.