can confirm after reading the replies here (https://protonmail.uservoice.com/forums/284483-proton-mail-calendar/suggestions/48146006-reply-to-emails-sent-with-hide-my-email-alias), that this actually is really working!

Choosing which email from your proton acc is REALLY misleading, hence I never dared to "test" this when doing an reply to a hide-my-email, so can only upvote the misleading visual that it seems that you send from your official proton email address. Even after sending a reply, it visually indicates that you sent it from your proton.me

The suggestion would be either display in FROM: the actual hide-my-email-address or if that is too complicated a "using your alias" would be also fine.

That would clear this up big time (as I up till now used the proton pass functionality to reply with the contact alias manually)