What am I missing about bonds?

Hey there. So I invest regularly into index funds following the bogleheads strategy (not exactly, but more or less). I use the formula "120 - your age" to allocate some money into bonds, currently using Vanguard Euro Government Bond Index (IE0007472115). My country offers transfers between funds without paying taxes, so they have this advantage over ETFs. What I don't really get is that this fund has had a -2.33% annualized return in the last 5 years, so I'm losing money here and there. I unders