Near the North/South poles, longitude lines converge to a point; planar buffering and equirectangular projections badly distort cell shape and area right at the pole cells.
▪ Rendered from the real conversion/geometry code.
Detection
Cell centroid or boundary latitude within a few degrees of ±90°.
Mitigation
Use geodesic (great-circle) buffering, never planar, near poles
Flag and manually review any cell whose boundary vertices approach ±90° latitude