[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?