LeetCode 79. Word Search problems storing previously visited

Question

My question is very similar to: leetcode 79 word search python solution, need help on debugging

The case that I'm having trouble with is the same:

[["A","B","C","E"],["S","F","E","S"],["A","D","E","E"]]
"ABCESEEEFS"

The problem I'm having is basically the same. The aforementioned question is answered, so I understand the principle idea of the problem (if I delete the part about checking previously visited, the code succeeds for this case), but can somebody explain step by step why my visited list is getting polluted?

class Solution:
    def exist(self, board: List[List[str]], word: str) -> bool:
        top = -1
        bottom = len(board)
        rowstart = -1
        rowend = len(board[0])
        def traverse(a, b, index, visited):
            print(a,b)
            print(visited)
            print()
            if index == len(word):
                return True
            if a == top or a == bottom or b == rowstart or b == rowend:
                return False
            if board[a][b] != word[index]:
                return False
            if [a,b] in visited:
                return False
            visited.append([a,b])
            return (
                traverse(a+1, b, index+1, visited)
                or
                traverse(a, b+1, index+1, visited)
                or
                traverse(a-1, b, index+1, visited)
                or
                traverse(a, b-1, index+1, visited)
            )
        for c in range(0, len(board)):
            for d in range(0, len(board[0])):
                if board[c][d] == word[0]:
                    if traverse(c,d,0,[]) == True:
                        return True
        return False