The theory about bonds say that when the interest rates raise, bonds value falls since today's bonds have a higher yield than yesterday's bonds.
However, let's take the 2018-2019 period. The FED raised interest rates from 1.5 to 2.5 as can be seen here.
However, the Vanguard's USD Treasury Bond UCITS ETF (see here) increased its value by quite a lot during the same period.
In other words, if an investor would have invested all his money 1 January 2018 in this ETF, it would have obtained a benefit selling on 1 January 2019, even though the FED raised interest rates 'a lot' during this period. Doesn't this contradict the theory?