I have had this problem for a long time and I do not know how to solve it, I have tried to change some rendering options but no visible effect appears at all, every time I try to Deny a part this appears and I don’t know if there is another way around to fix this.
Are you trying to Negate a rectangular Part, or a Union out of the box cover?
Also do you have other Parts/Unions behind that cover that are now showing through?
Another thing I see is that the box ‘cover’ isn’t what’s being selected after you make the Union. There seems to be a shorter Part highlighted in the last image.
your negation has 0 clearance gaps in it, try remaking it and make sure that all the parts in that negation actually overlap (0.001 should be enough, it won’t be noticable either)
The cover that is on top does not affect since it is 0.2 studs away from the box, the problem is that the unioned parts once converted to negative, weird artifacts appears like this:
Always make sure when Unioning overlapping Parts that they actually overlap or they’ll leave those cutouts.
When making Negated Unions I try as often as possible to start with 1 Part and ‘cut’ sections off it with Negated Parts rather than making it out of Parts and Wedges since placing a Part and Wedge or a Part and a Part together may create tiny gaps between the items that will show up on the final product.
Roblox Unions are Nasty for adding extra faces. If you use Blender and export your Union into Blender you’ll see what a mess they can be.