C# characters cannot be counted correctly (algorithm)

When I am trying to solve an ACSL algorithm question today, I encountered the following question:

> INPUT: There will be three integers representing the number of values to be found, the base to be used between 2 and 9 inclusive, and the starting value in the base given that will be no more than 16 digits.<br>
OUTPUT: For each set of 3 input values, output a base 10 number representing the number of
times the largest digit in the inputted base occurs in the sequence of numbers generated.

> EXAMPLE: If n=15, b=8, and s=2, the numbers generated are 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 20. The largest possible digit in base 8 is 7 which occurs 2 times.

I followed the instruction and this is the Minimum Reproducible Code:

    static void Main(string[] args)
    {
        Console.WriteLine(TheFunc(25,5,324));
        // it should be 24, but it outputs 10
    }
    public static int TheFunc(int n, int b, int s)
    {
        List<string> num = new List<string>();
        string temp = "";
        int SumCount = 0;
        for (int i = 0; i < n; i++)
        {
            num.Add(ChangeBaseEX(s + i, b));
        }

        foreach (var single in num)
        {
            temp += single;
        }

        SumCount = temp.Length - temp.Replace((b - 1).ToString(), "").Length;

        return SumCount;

    }
    public static string ChangeBaseEX(int num, int changeto)
    {
        string TheNumber = "";
        int RestNumber = 0;
        while (num > 0)
        {
            RestNumber = num % changeto;
            TheNumber = RestNumber.ToString() + TheNumber;
            num = num / changeto; //num/=changeto
        }
        return TheNumber;
    }

When I type 15, 8, 2, it can show the result correctly, which is 2. However, when I type 25, 5, 324, it outputs 10, which is incorrect(the correct output should be 24).
<h1>what I tried:</h1>

1:
I tried to determine if my ChangeBaseEX function is correct:

        Console.WriteLine(ChangeBaseEX(20, 8));
        Console.WriteLine(ChangeBaseEX(4000, 6));
        Console.WriteLine(ChangeBaseEX(22, 2));
        //24
        //30304
        //10110

According to Online base conversion tool, my base conversion is correct.

2:
I tried to print every value to determine if there are anything wrong with my counting, but it seemed that it is correct:

            //Console.WriteLine(s + i);
            //Console.WriteLine(ChangeBaseEX(s + i, b));
            //Console.WriteLine("=======");

3:
I tried to write the same code in many different ways. For example, I tried to change SumCount = temp.Length - temp.Replace((b - 1).ToString(), "").Length; into many other ways such as single.Length - single.Replace((b - 1).ToString(), "").Length;.

4:
I tried to use single step debugging, but I found nothing wrong.

<h1>What I want:</h1>
I debugged every suspicious step and I found nothing wrong. In that case, I want to know which step I get wrong to let it count incorrectly.

<br>
<br>

<!--(just in case, I found this post on meta.stackoverflow.com, and this question is proper)-->

You missed the fact that the third number is expressed in the indicated base. So 324 in base 5 is actually the decimal value 89.

You'll need to pass the value in as a string, convert it from Base-N to an integer value, then use that as the starting point of your number sequence.

For instance:

static int CountMaxDigitsInSequence(int count, int baseNumber, string startValue)
{
if (baseNumber < 2 || baseNumber > 9)
throw new ArgumentOutOfRangeException($"Invalid base number: {baseNumber}");
if (startValue.Length > 16)
throw new ArgumentException($"Start value too long: \"{startValue}\"");
// Convert the Base-N string to a starting value
int start = FromBaseN(startValue, baseNumber);
// Get digit to search for
char searchDigit = (char)('0' + baseNumber - 1);
// Or: (baseNumber - 1).ToString()[0];
// Counter
int result = 0;
// loop through the desired value list 
for (int i = 0, value = start; i < count; i++, value++)
{
// Convert current value to string in Base-N
string text = ToBaseN(value, baseNumber);
// Count instances of our search digit
foreach (var c in text)
{
if (c == searchDigit)
result++;
}
}
return result;
}
// Convert a number in Base-N to an integer
// Where:
//	2 <= baseNumber <= 9
static int FromBaseN(string value, int baseNumber)
{
if (baseNumber < 2 || baseNumber > 9)
throw new ArgumentOutOfRangeException($"Invalid base number: {baseNumber}");
int result = 0;
foreach (char c in value)
{
int digit = (int)(c - '0');
if (digit < 0 || digit >= baseNumber)
throw new ArgumentException($"Invalid digit '{c}' in value \"{value}\".");
result = result * baseNumber + digit;
}
return result;
}
// Convert an integer to a string in a paticular base.
// Where:
//	value > 0
//	2 <= baseNumber <= 9
static string ToBaseN(int value, int baseNumber)
{
if (baseNumber < 2 || baseNumber > 9)
throw new ArgumentOutOfRangeException($"Invalid base number: {baseNumber}");
if (value < 0)
throw new ArgumentException($"Invalid value: {value}");
string result = string.Empty;
while (value > 0)
{
int digit = value % baseNumber;
value /= baseNumber;
// This is a little dirty, but it works.
result = digit.ToString() + result;
}
return result;
}

And just for fun, here's a stripped-down ToBaseN() implementation using Span<char> to reduce the number of allocations.

static string ToBaseN(int value, int baseNumber)
{
Span<char> buffer = stackalloc char[17];
int index = 1;
while (value > 0)
{
buffer[^(index++)] = (char)('0' + value % baseNumber);
value /= baseNumber;
}
return new string(buffer[^(index)..]);
}