grid_boundary returns incorrect geometries for "NE", "NW", "SE", "SW" 100km BNG grid references

[tompeterken-os]

When using 100km-resolution BNG indices in grid_boundary, the tiles "NE", "NW", "SE", and "SW" are interpreted as being a "quadrant" resolution, so are erroneously being given geometries with half dimensions.

To Reproduce

import mosaic as mos
from pyspark.sql import functions as F

# Enable BNG indexing
spark.conf.set('spark.databricks.labs.mosaic.index.system', 'BNG')
mos.enable_mosaic(spark, dbutils)

# Create a dataframe with 4 100km grid references
df = (
  spark.createDataFrame([
    {'bng': g} for g in ['SE', 'SF', 'NX', 'NW']
  ])
  # Define boundary geometry
  .withColumn('grid_boundary', mos.grid_boundary('bng', F.lit('WKT')))
  # Measure area of each tile (should be the same for all)
  .withColumn('area', mos.st_area('grid_boundary'))
)

df.display()

All rows in the dataframe ought to have the same area of 10^10 m^2, but "SE" and "NW" do not:

Similar results for "NE" and "SW" too.