Expanding the table selection with the keyboard leaves the caret behind the selection's edges. After we pass the current cell content, the selection is automatically broadened over the next complete cell, but if we continue to press Shift+Arrow, it takes keystrokes = length of content in the cell + 1 to expand the selection to its next cell, though the selection highlights are already covering that cell.

There is a workaround: you will need to subscribe to the SelectionChanged event and use this code snippet for the event handler:

private void SelectionChanged(object sender, EventArgs e)
{
    var selection = this.radRichTextBox.Document.Selection;
    var caretPosition = this.radRichTextBox.Document.CaretPosition;

    if (selection.IsEmpty)
    {
        return;
    }

    if (selection.Ranges.Last.IsReversed)
    {
        caretPosition.MoveToPosition(selection.Ranges.First.StartPosition);
    }
    else
    {
        caretPosition.MoveToPosition(selection.Ranges.Last.EndPosition);
    }
}

Inside it, we move the caret to the selection's edges.

Insert a Plain Text Content Control to current document.

Enter a character in the Content Control.

Undo and then Redo.

The  Content Control remains in placeholder mode, which is not supposed to.

When a new line is placed right after a nbsp, the new line is ignored in the layout. This can be reproduced by importing the following HTML:

First&nbsp;<br/>Second