Why does curl -k -I https://host.example.com not return response headers if certificate is not valid?

Does anyone know why curl with -k (--insecure) option and -I for show headers still shows the html response and not the headers as expected?

Working as expected:

NOT working as expected:

It doesn't really matter here what I'm doing, but I'm testing what headers get set to identify different backend acl logic on haproxy.

I would expect curl to allow me to make an insecure connection (invalid certificate) and still return the headers?