Why are there offsets when rendering the same annotation style in adjacent slices in the vertical direction? #287

When developing a map service based on the TMS protocol using Python Mapnik and using the XML Config Reference rendering style, when the map level is very small, why is there an offset when two adjacent slices in the vertical direction are rendered with the same annotation style, as shown in the figure above, and cannot be correctly spliced ​​into a complete annotation?

This picture is the tile above。

This picture is the tile below。