There’s no indication a Microsoft appointed board member would be a Microsoft employee (though the they could be of course), and large nonprofits often have board members that come from for-profit companies.

I don’t think the IRS cares much about this kind of thing. What would be the claim? They OpenAI is pushing benefits to Microsoft, a for-profit entity that pays taxes? Even if you assume the absolute worst, most nefarious meddling, it seems like an issue for SEC more than IRS.