Proton does not use the Reply-to: header of the email.

We use a webform in the contact section. Emails are sent from site@customdomain.com to site@customdomain.com with a Reply-to: header of customer@someone.com
When pressing Reply, it should see the Reply-to: header and send to the customer. Instead if sees it's sent from site@customdomain.com and replies to site@customdomain.com

Gmail Reply-to: headers work fine with the same forms.