On the one hand, some bonds invested by bond funds may default on maturity, and the principal and interest cannot be recovered on time, which may lead to fund losses.
Secondly, even if there is no bond default, pure debt funds will fluctuate. There are coupon rate bonds, but bond funds have no fixed interest rate. Whether a bond fund goes up or down depends on its net value. The net value of the fund is related to the price of the bonds it holds. The bond price rises and the net value of the fund rises; When the bond falls, the net value of the fund falls.
The rise and fall of the net value of bond funds you usually see is meaningless, because they are all book values. Until you want to redeem it, the net value at redemption is relative to the net value at purchase. If it goes up, it will earn, and if it goes down, it will lose. The profit and loss during the holding period is just a cloud, just a process, not a result.