Steps to reproduce would be useful. Per the current design the tool shouldn't even offer this option.

@PWaigi-WMF given that: 1) the option to translate a lead section is not even an option right now -which makes the issue hard to experience-, 2) we have a plan (T345845) to support lead section translation for existing target articles -which will be the right opportunity to find a fix any related issue- and 3) we don't have steps to reproduce the issue, I'm suggesting to remove this task from our current sprint. We can revisit it and monitor the situation, once we deploy the code for T345845, or at least get some steps to reproduce.

Given that both edits linked in the task description have the tag "mobile edit", I used the Content Translation tool on my mobile bowser and could easily reproduce the bug @ngkountas. There's definitely an option to translate the lead section. Detailed steps to reproduce:

• Open Special:ContenTranslation while using the mobile Version of Wikipedia (I didn't check if the same issue also exists on desktop)
• Choose an article which already exists on your target wiki
• Make sure the translated title corresponds to the existing article title on your target wiki
• Translate the lead section
• Publish the translation
• The translated lead section appears below the existing article content

Example:

• https://en.wikipedia.org/wiki/Heirs_of_the_Night doesn't exist on German Wikipedia. But https://de.wikipedia.org/wiki/Die_Erben_der_Nacht does (about the book instead of the TV series).
• If you want to translate the article about the TV series and Special:ContentTrranslation https://de.m.wikipedia.org/w/index.php?title=Special:ContentTranslation&filter-type=automatic&filter-id=previous-edits&active-list=draft&from=en&to=de&page=Heirs%20of%20the%20Night#/sx/sentence-selector MinT will suggest to translate the title "Heirs of the Night" to "Die Erben der Nacht" without notifying you that there's an existing article with the same title. Accept the suggestion for this title (or insert this translated title manually).
• Translate the first sentence and click publish to a new page and you can make the same edit https://de.wikipedia.org/w/index.php?title=Die_Erben_der_Nacht&diff=prev&oldid=256412879 which was reported in the task description https://de.wikipedia.org/w/index.php?title=Die_Erben_der_Nacht&diff=prev&oldid=245372995

Additional bug: The mobile translation interface offers machine translations even though this should be disabled on German Wikipedia (and is disabled using the desktop version of Special:ContentTranslation).

Thank you a lot for all the information @Johannnes89! I have already created a new task about the issue with having machine translation available on mobile on German Wikipedia (T395422), which will be worked on soon. Regarding the other issue you described, about lead section translation I'll discuss it with our team and determine our next steps. I'll follow up here, once a decision is made.

This task is not yet ready to be worked on, since the required designs are not yet available. Moving the task back to the "Backlog" and adding the "Design" tag for visibility.