weird behavior with h3_polygon_wkt_to_cells()

[ericemc3]

buffer1_g.json
buffer2_g.json
buffer3_g.json

These are 3 buffers around river Garonne (1000, 2000 and 3000 meters).
I want to get H3 cells for these buffers.

FROM st_read('buffer2_g.json')
SELECT geom.h3_polygon_wkt_to_cells(8);	

returns an empty list, which is unexpected.

FROM st_read('buffer1_g.json')
SELECT geom.h3_polygon_wkt_to_cells(8);	

returns a list with 2 443 cells: OK.

FROM st_read('buffer3_g.json')
SELECT geom.h3_polygon_wkt_to_cells(8);	

returns a list with 5 479 cells: OK.