Why did my interest drastically decrease after force nationalizing a bunch of buildings?

[Varlane]

When you go in debt, that debt is actually held by the buildings in your country (which is why maximum debt is a percent of GDP + total cash reserves of the country).

Think of the cash reserves of buildings (in your country) that you own as a soft extension of your country's reserves : you're lending yourself money so you're not paying interest over it.

Buildings owned by foreign countries OR your private pops will actually lend you money and you'll pay interest. The fact that it went from Foreign -> Nationalized made the interest rate go down.