Ask them to print out their Tax Account screen, as proof, if they don't have their letters. And are you following the topic where the letters turn out not to match what the tax account is showing?

"although I don't think amended returns would be necessary here, IRS will catch and correct the error."

Yes, tell the clients, as was on the national evening news recently, that their potential 2021 refund delay due to an IRS correction or to filing an amendment likely will be similar to that delay right now; 6 million returns and 20 months out already.