There are still tests that test array elements by indexing its values one by one, which will throw an error if the returned value isn't indexable.

They should be replaced with array comparison.

Compound interest for fraction amount of unit of payout period is undefined in real life: Real life interest are paid in specific period (e.g per day/month/year). The kata assumes infinitely fine-grained interval, but it's only a mathematical construct; you can only assume interest is multiplicative and analytically extend the values to all real values between the payout periods (which results in exponentials), but it's a limit.

And all of this was never explained anywhere, so it's only up to pure guessing what payout model is being used by the kata.

Is the cycle of the interest calculate related to the calculation?

Or the cycle of the interest calculate is one day?