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I think what you may be missing here is the fact that bonds that are inflation-indexed (eg TIPS) pay a fixed coupon on a face value that adjusts according to inflation. This means that investors do not face purchasing power erosion upon return of principal value.
In the case of a non-inflation indexed bond that pays a nominal rate of interest, in the event inflation (or expected inflation) changes, investors will continue to receive the same coupon and principal value upon maturity and it will be worth less in real terms.