1. I think I only got one grid that was unsolveable.

2. That was my first grid.

3. I was only playing with addition.

4. I completed exactly one round.

Because the "unsolveable" grid was my very first, I may well have pressed on without investigating too much, because perhaps things would become clearer; and indeed, all subsequent grids were easy to solve.

So please don't treat my comment as a bug report! If you've taken trouble to avoid that outcome, then almost certainly the error is me.