Big bond dealers are becoming less relevant as more investors and institutions buy directly from the Treasury.
The flattening has been driven by the hawkish stance of the Federal Reserve, according to one strategist.
That's because the Fed's Treasury holdings include inflation-protected securities.
The Treasury Department over the past year has deeply cut the size of its 20-year auctions.
The liquidity in Treasury Inflation-Protected Securities has made notable strides.
So-called inversions of the yield curve are regarded as a potential harbinger of recession.
The last persistent inversion of the Treasury curve occurred in 2006-2007, though it briefly inverted in 2019.
In December, the government sold $2.78 billion of Series I savings bonds, which pay a fixed interest rate plus inflation.
Jerome Powell, who has been in the job since 2018, remains the betting-market favorite to continue in the role.
Price inflation now significantly exceeds wage inflation, and bond markets predict it will get worse.