HTML Mails from one Domain are displayed incorrectly

Steps to reproduce

All mails received from heise.de 1 are malformed in the mail app:

Expected behavior

I expect a readable view of the e-mail.

Actual behavior

See picture above.
Edit: Snappy Mail (on the same Nextcloud instance) and Roundcube (E-Mail service provider) display the mails correctly. The mails are also displayed correctly after exporting the mail from the mail app and opening with Outlook or Thunderbird.

Mail app version

3.3.1

Mailserver or service

Own mail server

Operating system

Ubuntu 22.04.3 LTS

PHP engine version

PHP 8.1

Web server

Nginx

Database

MySQL

Additional info

As requested by kesselb @ help.nextcloud.com please find an original mail from heise.de, which was downloaded from the mail app (sensitive information was replaced - added .txt extension as .eml would not upload)

Mittwoch Google macht Kasse mit Kartendaten, Nord Stream-Spur in die Ukraine.eml.txt

Start by reproducing the issue in the mail app with the attached “Mittwoch Google macht Kasse mit Kartendaten, Nord Stream-Spur in die Ukraine.eml.txt” message and inspect how its HTML is rendered. Compare the result with Outlook, Thunderbird, Snappy Mail, or Roundcube; done means the heise.de message has a readable view in the app.