RichTextBox interprets the div tag by adding a new line upon its closing. However, when the content of the div tag is a br tag, the whole combination should be treated as one new line.
Hi Lan, The item for the issue in the Winforms control can be found here: https://feedback.telerik.com/Project/154/Feedback/Details/252172-fix-radrichtexteditor-br-tag-enclosed-in-a-div-should-be-treated-as-a-single-n
Same issue on Richtextbox for Windows Forms