I guess the same way email can have html as an attachment for the same thing a plaintext does, evidently some of these mails suggested a mailer actually pdf encoded the email and attached, as well as the plain text.

So when someone replied with plaintext the base64 encoded PDF that they were replying to got ‘quoted’, meaning the unredacted email they were replying to is in there, just messy due to font confusion in the provided format.