It's actually written on the treasury page
Negative Yields and Nominal Constant Maturity Treasury Series Rates
(CMTs): At times, financial market conditions, in conjunction with
extraordinary low levels of interest rates, may result in negative
yields for some Treasury securities trading in the secondary market.
Negative yields for Treasury securities most often reflect highly
technical factors in Treasury markets related to the cash and
repurchase agreement markets, and are at times unrelated to the time
value of money.
At such times, Treasury will restrict the use of negative input yields
for securities used in deriving interest rates for the Treasury
nominal Constant Maturity Treasury series (CMTs). Any CMT input points
with negative yields will be reset to zero percent prior to use as
inputs in the CMT derivation. This decision is consistent with
Treasury not accepting negative yields in Treasury nominal security
auctions.