line indenting does not work

Using the line indenting button of the WYSIWYG editor does not work for standard paragraphs.
The editor obviousely does something: The line is (a bit), and for each indenting level, one vertical blue bar is displayed next to the line.

However, if you save the text, not only are the lines not indented, but they are also concatenated.

How to reproduce:

• create a new document
• enter three lines ("1", "2" and "3")
• for each line, move the cursor to that line and press the indent buttton
• save the page
• the text is now displayed as "123"