Default text behaviour in a DataGridViewTextBoxCell control is that it doesn't wrap.
The behaviour can be changed using the WrapMode property on the cell style. For example, DataGridView.DefaultCellStyle.WrapMode.
This can cause that new line characters in the text do not apply and that's the reason why they are displayed as a non-printable characters. You can compare this behavior with Text property of a TextBox control when the MultiLine property is set to "false".