@Andre-Radtke In order for the HTML to text action to recognize and preserve these line breaks, there needs to be HTML tags such as <br> at the end of each line or for each line to be wrapped in a <p></p>.
Unfortunately in the screenshot you've shared above, although it might look like there are line breaks separating each line of text—there isn't any html that marks the end of a line.
For example, it should look something like this:
<p>2023-M008 Projectstart<br>
2023-M009 Defining project scope<br>
2024-M002 End of preparation work<br>
....
or
<p>2023-M008 Projectstart</p>
<p>2023-M009 Defining project scope</p>
<p>2024-M002 End of preparation work</p>
....
In a previous post, you indicated the output was:
<div class="ExternalClassDE3B8EFA35C24BD1A85839CF8C7053F5"><p><div class="ExternalClassA106B74B640D44E3865A20E67875B3BF">Defektes Kabel</div></p></div>
The HTML to Text action should output:
Defektes Kabel
In SharePoint do you have this toggle set to Yes or No?

If set to no, the output of the Multi-Line text field will be a single paragraph (<p></p>) tag.

However, if you have the toggle set to Yes, the output may look like this. Keep in mind, this also depends on how the text is entered into the multi-line text field.

Hope this helps.