I'm quite sure that debts are paid in any case because not paying them would be a disaster for the bond issuer.
The procedure in times of war, at least by the US treasury is to put the payments in a blocked account that is then given to the citizen of the opposing country post war.
This is also why russian assets in US/EU banks, e.g., are only frozen: doing any other thing with them would instantly trigger the entire world to liquidate their holdings. Would Saudis or Indians or Danes hold assets that can be confiscated for geopolitical reasons?
Not quite... US can freeze certain assets. It did that for Iran for example. It froze the assets for a few decades (from 1979 until 2016). In 2016 it paid back the principal and interest: https://www.brookings.edu/articles/the-united-states-iran-an...