This must be a bug, right?
It’s a little bit more complicated than only being on surplus. As You can see - The global economy is also falling down
I’m not convinced by this.
Here is a screenshot from the same save. The credit rating has remained the same over the last two election cycles of 4 years. It was finally increased as my surplus has slowly grown over the last few quarters.
This is intriguing, even if the global economy is going down, your surplus is still present and you are still paying off your debt. Maybe it is in anticipation of your economy receding that the credit interest has gone up. Is this effect still present in v1.34? That would help us to find out, as the CR system has been revamped.