Git submodule update remote keeps using previous default branch

[Sjoerd Langkemper <sjoerd-2021@linuxonly.nl>]

I am having trouble with git 2.33.1 checking out the wrong branch for
submodules for which the default branch has changed. `git submodule
update --remote` seems to remember the branch name to retrieve, while
I expect it to use the remote HEAD every time. This causes unexpected
behaviour when the remote HEAD starts pointing to another branch.

I create a new git project and add a submodule, with `git submodule
add git@host:foo/testproject.git`.

This checks out the default branch, `master` in this case. `git remote
show origin` also shows that `master` is the HEAD branch. Running `git
submodule update --remote` updates the submodule to the latest master.

Now I change the default branch on the remote (using Gitlab's web
interface) to `newmaster`.

`git remote show origin` now correctly shows `newmaster` as the
remote's HEAD branch. However, running `git submodule update --remote`
still updates the submodule to the latest `master` branch, while I
expect it to update to the lastest `newmaster` branch.

There's no branch specified in .gitmodules or .git/config. I am not
sure how git remembers the branch. When switching `testproject` to
`newmaster` manually and then running `git submodule update --remote`,
it is reset to `master` again.

Is this a bug? Can I change the branch somehow?

Regards,

Sjoerd Langkemper