In RichText Editor, you can create line breaks (inside a paragraph) by pressing Shift + Enter.
If line breaks are created inside the paragraph that doesn't have any formatting, then line breaks are properly saved.
If line breaks are created inside a paragraph that is formatted (for example made bold) as a whole, then line breaks are removed after saving/publishing.
If line breaks are created inside a paragraph that doesn't have any formatting, and then each line is formatted separately (for example made bold), then line breaks are properly saved.
This bug is not present in eZ Publish.

Steps to reproduce

1. Create new eZ Platform installation.
2. Go to the Platform UI. Create new article there.
3. In one of the article's Rich Text fields (for example "Intro"), enter few lines of text. Lines should be inside one paragraph and separated by line breaks (Shift + Enter).
4. Select the entire just created paragraph and make it bold by clicking the "B" icon.
5. Publish the article.
6. Edit the article again. Notice that the line breaks are gone.