eml not attached when "mark as spam"

Steps to reproduce

Some emails when marked as spam are not being attached when forwarding to my spam@mydomain.com.

I'm not really sure why with some emails it works ok but with some others doesn't.

I think it could be related with some odd characters or emojis on the subject, but I couldn't always reproduce.

Expected behavior

I was expecting that after eml name, the actual eml content

Actual behavior

See this example:

Content-Type: multipart/mixed; boundary="=_pFyz-xNmOnKyMMkP2g2Bn6R"
MIME-Version: 1.0

This message is in MIME format.

--=pFyz-xNmOnKyMMkP2g2Bn6R
Content-Type: multipart/alternative; boundary="=-uOQk8wOCf0_vv_q92EICfx"

This message is in MIME format.

--=_-uOQk8wOCf0_vv_q92EICfx
Content-Type: text/plain; charset=utf-8

Learn as Junk
--=_-uOQk8wOCf0_vv_q92EICfx
Content-Type: text/html; charset=utf-8

Learn as Junk

--=_-uOQk8wOCf0_vv_q92EICfx--

--=_pFyz-xNmOnKyMMkP2g2Bn6R
Content-Type: message/rfc822;
name0=utf-8''Hac%C3%A9%20tus%20env%C3%ADos%20m%C3%A1s%20f%C3%A1ciles%20y;
name1=%20r%C3%A1pidos%3A%203%20pasos%20para%20lograrlo.eml

--=_pFyz-xNmOnKyMMkP2g2Bn6R--

Mail app version

5.7.15

Nextcloud version

33.0.3

Mailserver or service

dovecot

Operating system

ubuntu 22.04

PHP engine version

PHP 8.3

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

I understand that attaching the full email message will always have all the needed information. but as a mail server admin a simplified (and truncated) version with some fields would be better to have, like:

DateTime:
From:
To:
CC:
Reply-To:
Subject:
Truncated Body:
Size:
Attachment names:

It could be json encoded and be sent by email.

Start by reproducing the “mark as spam” forwarding flow with the supplied MIME example, then trace where the EML attachment content is generated. The fix is complete when affected messages, including subjects with encoded characters or emojis, are forwarded with the full EML content attached.