For all the dire warnings over China's retreat from U.S. government debt, there's one simple fact that is being overlooked: American demand is as robust as ever.

Not only are domestic mutual funds buying record amounts of Treasuries at auctions this year, U.S. investors are also increasing their share of the $12.9 trillion market for the first time since 2012, data compiled by Bloomberg show.

The buying has been crucial in keeping a lid on America's financing costs as China — the largest foreign creditor with about $1.4 trillion of U.S. government debt — pares its stake for the first time since at least 2001. Yields on benchmark Treasuries have surprised almost everyone by falling this year, dipping below 2 percent last week.