Attached very simple file showing problem. It is a cylinder fitted exactly into a hole in a cube. The faces of the two objects are in the same place. Slicing is completely wrong. This is a new bug. This did not happen in around 1.79.
Overlapping objects have always been supported in Chitubox, it is very convenient because you don't have to merge objects in your 3D art package. However, sometimes overlapping objects have exactly the same faces, pointing in different directions. It seems to fail when one object completely surrounds another(?)