It is possible that one of the tiles will not be refreshed without marking it as dirty like SomeoneElse did. This is possible when the algorithm does not realise the content of the tile has to be refreshed, maybe this is the case here, as the actual point that has changed is in the top tile, the bottom tile might not know that it has to be re-rendered.

I’m not familiar with those kinds of details of the algorithm, so I can be totally wrong.