Changes Content-Transfer-Encoding from 8bit to quoted-printable

I've just tried out to disable header/footer/prefix in mailman and now it often can forward messages without modifications. This basically resolves problems with DKIM/DMARC.

But then someone wrote a message with "Content-Transfer-Encoding: 8bit". Mailman3 changed it then to "Content-Transfer-Encoding: quoted-printable" (which would already break DKIM) and adjusted the body to encode things like spaces on the beginning of lines (for patches!!!) to =2D.

I wasn't yet able to find where this actually happens. https://bugs.launchpad.net/mailman/+bug/373083 sounded like it is in handlers/decorate.py. But when I add a return directly at the beginning of the process function, it still causes this kind of transform.

Is there some kind of workaround for this or is this a known problem?