[BUG] Amending a shallow clone -> orphan branch

Consider the following steps :

git clone --depth 1 --single-branch --branch master https://github.com/git/git
cd git

1.txt

git add 1.txt
git commit --amend

This results in an orphaned branch, where a single commit contains entire tree.

I understand that this is a bug, because certainly git knew shallow commit's parents and could reuse that when amending?