"sa-learn --forget msg" gives the following warning

message: _normalize() was given characters, expected bytes: darkThemeSupport=true backgroundImageSupport=true headerImage

on the message (which I've reduced to the following lines):

Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset="UTF-8"
MIME-Version: 1.0

pener"> D=C3=A9couvrir d'autres =C3=A9v=C3=A9nements </a></td></tr></table>=
</tr></table><![endif]--></td></tr></tbody></table></div><script type=3D"te=
xt/plain">darkThemeSupport=3Dtrue backgroundImageSupport=3Dtrue headerImage=

I've also reported the bug upstream.

Note: the --forget is just an example; it also occurs with --spam.

This is annoying since I run sa-learn via cron, and I get spammed
each time it is run. However, I could remove the culprit message
(which I identified with the -D option).