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I'm trying to generalize World Bank Official Boundaries:World Boundaries GeoJSON - Low Resolution from https://datacatalog.worldbank.org/dataset/world-bank-official-boundaries using QGIS 3.18.3, using GRASS v.generalize, ultimately to display data using vega-lite, but some shapes aren't generalizing

Firstly, I removed the "fid" field, since that fixes issues about unique constraint violations. Then, I run the v.generalize with all the default options, which are

{
  '-l' : True,
  '-t' : False,
  'GRASS_MIN_AREA_PARAMETER' : 0.0001,
  'GRASS_OUTPUT_TYPE_PARAMETER' : 0,
  'GRASS_REGION_PARAMETER' : None,
  'GRASS_SNAP_TOLERANCE_PARAMETER' : -1,
  'GRASS_VECTOR_DSCO' : '',
  'GRASS_VECTOR_EXPORT_NOCAT' : False,
  'GRASS_VECTOR_LCO' : '',
  'alpha' : 1,
  'angle_thresh' : 3,
  'beta' : 1,
  'betweeness_thresh' : 0,
  'cats' : '',
  'closeness_thresh' : 0,
  'degree_thresh' : 0,
  'error' : 'TEMPORARY_OUTPUT',
  'input' : '/home/dw/Desktop/WB_countries_Admin0_lowres.geojson',
  'iterations' : 1,
  'look_ahead' : 7,
  'method' : 0,
  'output' : 'TEMPORARY_OUTPUT',
  'reduction' : 50,
  'slide' : 0.5,
  'threshold' : 1,
  'type' : [0,1,2],
  'where' : ''
}

I get a good amount of warnings in the output, but it does make a new layer

WARNING: Number of incorrect boundaries: 2205

WARNING: The combination of types is not supported by all formats.

WARNING: No attribute table found -> using only category numbers as attributes

WARNING: 3668 features without category were skipped. Features without category are written only when -c flag is given.

WARNING: Output layer is empty, no features written

WARNING: 2237 boundaries were not modified because modification would damage topology

WARNING: 3638 lines/boundaries were not modified due to over-simplification

Spain looks generalised, but France and the UK, no. (Technically Spain is more generalized than I want, but expect that's a matter for tweaking the tolerance)

enter image description here

I can sort of understand this from the warnings... but what I don't know: what to do about it. How can I get all the countries to generalise? I would like the boundaries of countries to remain aligned without gaps appearing between them.

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