Password protecting a Proton alias email revealed my actual Proton email address

I needed to send sensitive documents to a business. Their system blocks encrypted files so that wasn’t an option and we were struggling to come up with a way they would accept it. I guess I’m the first person they interacted with that cares about encrypting sensitive files. They agreed to accept the password protecting email that Proton offers . I have a free Proton account currently and for this business, I use a Proton Pass alias. Since I’m paranoid and wanted to make sure it worked properly, I tested this by first sending an email from a non-Proton email to the alias. I then responded back, enabling the External Encryption button. The email I received in the non-Proton inbox does say it came from the alias. But, the email body itself says “You have received an encrypted email from [my actual Proton email address].” So…yeah…ummm..WTF?!!?