Hi milkywaif, the zbias amount (amount of pull forward), is set on an individual object basis based on its bounding box size. So the small object has a smaller zbias pull associated with it, once it's booleaned those edges will then use the zbias amount based on the bounding box of the whole large object.

In your particular case there it needs a higher amount of zbias for the display artifact to go away. But unfortunately just increasing zbias up for everything instead causes a different artifact where stuff that's supposed to be hidden will bleed through.

- Michael