Of course I can't find an example of failure, your interpretation of the conditions was very correct.

It's great, I think it's fantastic that you work at bit level.

I used detection of zero as a shortcut to get out of the routine, but you saved lines.

On the other hand I had to check that << -1 actually works, I always expected that offset would have to be a positive quantity. I learned something new for me.

Interesting board[i][j] = 0

x |= 1 << (board[i][j] - 1);
x |= 1 << ( - 1);
x |= 0

so nothing happen, 0 is ignored. It's great, maybe unplanned but great. The board fails due any digit missing