These bonds were originally issued with positive rates and the rates don't change just the yield does.
Made up Example; 20 year bond issued in 2010 for 1,000 with 5% return. Investor gets a $50 every year for 20 years then gets his 1,000 back. If demand is high for these bonds the price rises and say someone today pays 2,000 for the bond. They still get 50 every year and in 2030 get the 1,000 back. Thus this person loses money paid 2,000 and received 1,000 face plus 750 in dividend for a loss of 250.
It is tulip mania all over.
I would guess the government is buying these "losing" bonds in order to force investors to move towards the new long term issued bonds allowing the government to increase capital.