@mdmc
This is a common problem. We hear it a lot, here in the forum. It happened to me personally. But you are the first person to report it happening twice. As you know there is simply no where on the IRS forms to show that*, There's nothing you can do to prevent it. Keep good records so you can respond.
At least two users have reported receiving a CP2000 letter, from the IRS, on 529 distributions. They replied that their child was in college and the distributions were for qualified expenses, which they listed, but they did not provide receipts.. They later received a notices saying they were in the clear. In my case, I just sent em copies of the school invoices.
*In my case, I forced an entry on to line 8 of schedule 1 (where the taxable portion of a 1099-Q would be reported), and it still didn't prevent getting an IRS inquiry.
