Delete Merged Branches also deletes protected branches

Summary

Since 9.3.0, I suppose, the delete merged branches button delete also protected branches.

It is not reported in the documentation, and it is quite an important change. I do not see any comment on that in the changelog, so I suppose it is by mistake. See also https://gitlab.com/gitlab-org/gitlab-ce/issues/33445#note_33072180

What is the current bug behavior?

Delete Merged Branches button also deletes protected branches

What is the expected correct behavior?

Delete Merged Branches do not deletes protected branches, as per documentation.