Thanks, that makes sense.
So in this case the sudden change Andro is seeing comes from %{email_preview} being added recently (PR #36657), which explains why it appeared overnight without any admin changes.
From an admin point of view the pain point is similar either way: it’s core-injected content at the top of the email body, and there’s currently no site-wide toggle to disable or reposition it. The only workaround today is editing the affected email templates and removing or moving %{email_preview} (just like %{header_instructions}).
For hosted customers in particular, that’s why this feels abrupt — defaults changed, but there’s no supported global control.