To reproduce: run the attached sample project and activate the editor for the cell. You will notice that the row's height is now adjusted and single line text is displayed. It seems that the issue occurs because of the set Font property of the grid.

Workaround:  
        private void RadGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
        {
            e.Row.MinHeight = e.Row.Height + 3;
            RadTextBoxEditor tbEditor = e.ActiveEditor as RadTextBoxEditor;
            if (tbEditor != null)
            {
                tbEditor.Multiline = true;
            }
        }

        private void radGridView1_CellEndEdit(object sender, GridViewCellEventArgs e)
        {
            e.Row.MinHeight = 0; 
        }