hole in model disappears when sides are filleted

```
from cadquery.occ_impl.shapes import *

base = face(rect(50, 50)) - face(rect(40, 40))
base = base.fillet2D(5, base.vertices('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.