The DocumentContentChanged event is not fired when table cell border style is changed in RadRichTextBox. Resolution: This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.
This issue affects many other table formatting operations. I've experienced it with Table Padding or Spacing, Inserting pr Removing Rows or Columns, Cell color etc. The fix for this issue should address all types of table formatting.