Note that when you add a new line to your string, the elements on the new line will be moved into a new paragraph. Without your set up I can't be sure if this is the issue, but it seems that the line spacing is applied only on the first paragraph created by you, but the automatically created one (after the line break), uses its default spacing setting.
In order to achieve your requirement you can try editing the default normal style of the paragraphs. This way the spacing will be set globally.
var normalStyle =
normalStyle.ParagraphProperties.LineSpacingType = LineSpacingType.Exact;
normalStyle.ParagraphProperties.LineSpacing = 30;
If this doesn't help you can share more details of your set up. For example, where you change the document? How you set the new line? How is your view model defined? What is the set up of the data provider and the rich text box control?
Telerik by Progress
Telerik UI for WPF is ready for Visual Studio 2017 RC! Learn more.