I wrote this function below in Python and I am expecting to see 4 as the result but get 2. Why is that?

Here is my function:

def persistence(n):
    if 0 <= n <= 9: 
        return n
    else:
        result = 0
        counter = 0
        while True:
            result += n % 10 * persistence(n // 10)
            counter += 1
            if result > 9:
                n = result
                result = 0
                continue
            return counter

print(persistence(999)) // Output should be 4 but gives me 2

I know where the problem exactly happens. It is in the line in which I wrote result += n % 10 * persistence(n // 10). when I debug it in the first iteration, instead of giving me the number 729 (the multiplication of 9 * 9 * 9) it gives me 18. Why is that? What is wrong with my code that instead of getting 729 as the output for result in the first iteration I get 18 and therefore the value of persistence(999) is not what I expect it to be?