What is wrong with this flood fill algorithm?

Question

https://www.kakaocode.com/tryouts/1353/intro

Given a picture array, the code should return the number of separated areas and size of a maximum area.

The color is represented as an integer (0 for white. white area doesn't count).

m and n are # of row and col each.

For example,

m = 6, n = 4 
1 1 1 0
1 2 2 0
1 0 0 1
0 0 0 1
0 0 0 3
0 0 0 3

should return [4, 5] because there are 4 separate areas and the largest area contains 5 ones.

Another example:

1 1 1 0
1 1 1 0
0 0 0 1
0 0 0 1
0 0 0 1

this one should return [2, 6]

public int max = 0;
public int[] solution(int m, int n, int[][] picture) {
    // answer[0] = numberOfArea
    // answer[1] = maxSizeOfOneArea
    int[] answer = new int[2];

    for (int i = 0; i < m; i++){
        for (int j = 0; j < n; j++){
            if (picture[i][j] != 0){
                max = 0;
                floodFill(i, j, picture[i][j], picture);
                if (max > answer[1])   answer[1] = max;
                answer[0]++;
            }
        }
    }
    return answer;
}

public void floodFill(int i, int j, int num, int[][] picture){
    if (i < 0 || j < 0 || i >= picture.length || j >= picture[0].length || picture[i][j] != num)
        return;
    max++;
    picture[i][j] = 0;
    floodFill(i - 1, j, num, picture);
    floodFill(i + 1, j, num, picture);
    floodFill(i, j - 1, num, picture);
    floodFill(i, j + 1, num, picture);
}

I tried multiple test cases and it passed all, but it failed when I submitted.

What are some edge cases that I am missing?