Hi mir4ea - when 2 "analytic" planes are the things being unioned, it will form a larger plane and remove the interior edges. If you have objects that are not planar but just happened to be squished flat then that processing won't happen.

The other case where the edges can be removed is when the 2 trimmed surfaces being unioned have the exact same underlying surface, like if they were originally trimmed or booleaned out of the same surface.

- Michael