I think my phrasing was poor. If Ukraine is conquered then the government of Ukraine becomes pro Russian. That debt would remain. It would be a burden on the pro Russian government.
And given the Ukraine would have no revenue, the funding for that government would be coming from Moscow. Demands for payment by the creditors can be ignored, but the interest will continue to accumulate.
A fundamental truism would unfold. There is no such thing as sovereign default. A country that owes money continues to owe money even if they declared default. The creditors have to agree to reduce the debt for it to be expunged.
Just happened in Argentina and to an oblique extent, Greece too. There is no sovereign default. That cannot disappear from a country’s balance sheet unless the creditor agrees because there is no such thing as a sovereign bankruptcy Court, and empowered to expunge debt.
Why will the pro-Russian government recognize the debts of prior regime? Why shouldn’t it have an income? Check out the history of post-Cold war Ukraine. Its economy was booming under the neutral governments, but nosedived under pro-Western coupsters.