branch w/o depth parameter causes failure - "fatal: reference is not a tree"

Issue: if "branch:" parameter is a custom, not a master and "version: ref:" is not specified with proper revision, and if master contains fresher commit then the branch, $ref will pick up and shallow a commit which HEAD of master.
In such case, it will be incorrect revision taken by Concourse which then causes a failure with the message:
"fatal: reference is not a tree". See example:

Cloning into '/tmp/build/get'...
fatal: reference is not a tree: <ref>_

Looking at https://github.com/concourse/git-resource/blob/master/assets/in
the answer can be partially explained by the piece of code which is deciding what needs to be checkout - there is no $branch specified and just $ref used which is HEAD, if depth > 0 :

if [ "$depth" -gt 0 ]; then
  "$bin_dir"/deepen_shallow_clone_until_ref_is_found_then_check_out "$depth" "$ref" "$tagflag"
else
  git checkout -q "$ref"
fi

As a workaround setting flag "depth: " helps in case of "branch" parameter is custom (not a master).
Could you please explain behaviour, if anything can be done to fix this behaviour?

Start with assets/in and the deepen_shallow_clone_until_ref_is_found_then_check_out entry point, then reproduce the custom-branch case without a version ref while master has a newer commit. Trace how branch, ref, and depth determine the checkout target; done means the scenario no longer produces “fatal: reference is not a tree” or its expected behavior is clearly documented.