head_ref for issue_comment event on PR wrong AFAIK

[zdenekpizl-foxdeli]

Greetings,
when creating a PR, head_ref gets expected value. On the contrary, when commenting on PR, the head_ref is set to 'refs/remotes/origin/master' apparently. But I would expect the branch name or something like 'refs/pull/${{ github.event.issue.number }}/head'.

What did I get wrong? Thank you, ZP.