hole in model disappears when sides are filleted

[bigfighter]

from cadquery.occ_impl.shapes import *

base = face(rect(50, 50)) - face(rect(40, 40))
base = base.fillet2D(5, base.vertices('<X')) # A
base = extrude(base, (0, 0, 5))
#base = base.fillet(0.5, base.edges('>X')) # B

from cadquery import version
print(version('cadquery'))

base # display in jypyter

if neither A nor B is commented out, then the hole disappears from the block. is this a bug?

I'm running cadquery in docker.