Bonds only have interest when they mature, but this income must be reflected in every day, that is, allocated to every day.

There are transactions in the bond market, and the inter-bank bond market buys and sells bonds, just like the stock market.

Then buy and sell, and naturally there will be ups and downs in supply and demand.

Bonds have fixed income at maturity, but transactions involved before maturity will also depreciate and appreciate.

The reason for the decline is that bonds fall in the trading market, which is caused by trading and has nothing to do with the income when bonds expire.