Mail gets corrupted when using "forward" but not using the "reply" button

[KevinBLT]

For me, it seems like "quoted printable" encoding is ignored and the content is inserted as 7bit or similar - but only using "forward" from the ui (see screenshot). This will not happen by clicking reply (which should be more or less the same?)

Screenshot:

The recieved mail is composed as follows:

[...]
Content-Type: multipart/mixed; boundary="=part-mixed-irVzrrizqIfPKYwt"
[...]

--=part-mixed-irVzrrizqIfPKYwt
Content-Type: multipart/alternative; 
    boundary="=part-alternative-HQmbODIqpdIIhOG6"

--=part-alternative-HQmbODIqpdIIhOG6
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"

PLAIN TEXT VERSION
--=part-alternative-HQmbODIqpdIIhOG6
Content-Type: multipart/related; 
    boundary="=part-related-LXAvm92Qx6zMYqfM"

--=part-related-LXAvm92Qx6zMYqfM
Content-Type: multipart/mixed; 
    boundary="=part-mixed-html-k9gbrsCXPhTvmUHb"

--=part-mixed-html-k9gbrsCXPhTvmUHb
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset="utf-8"

HTML VERSION

--=part-mixed-html-k9gbrsCXPhTvmUHb--

--=part-related-LXAvm92Qx6zMYqfM
Content-Type: image/png; name="logo.png"
Content-Description: "logo.png"
Content-Disposition: inline; filename="logo.png"; size=17635;
Content-ID: <logo.png>
Content-Transfer-Encoding: base64

THE IMAGE INCLUDED BY CID

--=part-related-LXAvm92Qx6zMYqfM
Content-Type: image/jpg; name="banner.jpg"
Content-Description: "banner.jpg"
Content-Disposition: inline; filename="banner.jpg"; size=401671;
Content-ID: <banner.jpg>
Content-Transfer-Encoding: base64

ANOTHER IMAGE BY CID

--=part-related-LXAvm92Qx6zMYqfM--

--=part-alternative-HQmbODIqpdIIhOG6--

--=part-mixed-irVzrrizqIfPKYwt--

If someone with access and knowledge could post this into BugZilla I would be thankful. I am not a user of thunderbird myself, but I wanted to post this on behalf of a customer who is.