N-Queens : value of variable changing for no reason?

I have the following code

def poss_move(val, board, n):
  # val is a value between 0 and n*n-1
  for i in range(n*n):
    if i == val:
      board[i // n][i % n] = 'Q'
      continue
    if i % n == val % n or i // n == val // n: #mark row and column with a dot
      board[i//n][i%n] = '.'

    #mark diagonals with a dot
    if i % (n + 1) == val % (n + 1) and abs(i % n - val % n) == abs(i // n - val // n): 
      board[i//n][i%n] = '.'
    if i % (n - 1) == val % (n - 1) and abs(i % n - val % n) == abs(i // n - val // n):
      board[i//n][i%n] = '.'
  return board

def next_available_position(val, board, n):
  if val < 0 or val >= n * n:
    return -1
  for i in range(val, n * n):
    if board[i//n][i%n] == '0':
      return i
  return -1

def sol(val, board, bt, result, n):
  board[val // n][val % n] == 'Q'
  print(f'AW1 val: {val}, bt: {bt}')
  board = poss_move(val=val, board=board, n=n)
  print(f'AW2 val: {val}, bt: {bt}')
  nap = next_available_position(val=val, board=board, n=n)
  while nap >= 0:
    board_tmp = [i for i in board]
    result, board_tmp = sol(val=nap, board=board, bt=board_tmp, result=result, n=n)
    board = [i for i in board_tmp]
    board[nap // n][nap % n] = '.'
    nap = next_available_position(val=nap, board=board, n=n)
  result.append([board,2])
  return result, bt

def run():
  m = 3 #board size = n x n
  board_ini = [['0' for x in range(m)] for y in range(m)] #initialize board with '0' in every row and col
  result = []
  board_t = []
  result, board_t = sol(val=0, board=board_ini, bt=board_t, result=result, n=m)

run()

I am getting the following output:

AW1 val: 0, bt: []

AW2 val: 0, bt: []

AW1 val: 5, bt: [['Q', '.', '.'], ['.', '.', '0'], ['.', '0', '.']]

AW2 val: 5, bt: [['Q', '.', '.'], ['.', '.', 'Q'], ['.', '.', '.']]

My question is: why is the value of bt variable changing from:

AW1 val: 5, bt: [['Q', '.', '.'], ['.', '.', '0'], ['.', '0', '.']]

to

AW2 val: 5, bt: [['Q', '.', '.'], ['.', '.', 'Q'], ['.', '.', '.']]

I am getting nuts, thought it was something about using global variables so I rewrote all the code to avoid them as far as I understand bt is a local variable and I am doing nothing to it between the 2 prints and its value is changing.