It's hard to understand what is going wrong without a sample. Can you prepare and send us one? Do you have the same problem with other fonts that support your local characters?
In iGrid.NET, we use the classic Graphics.DrawString(text, font, brush, rectangle, stringFormat) method to output the cell text. Can you try to use it with your font/text? If youhave the same problem, then it is a bug in the .NET implementation of this routine.
P.S. iGrid.NET is a WinForms control, but not an ActiveX one. ActiveX and WinForms are two completely different things.