I'm having this exact symptom.  I have an STL file which, when sliced vertically (my preferred method of printing) I get a single layer of print joining two parts of the geometry about halfway up the model.  This plane of unexpected print remains parallel to the bed and doesn't appear in every version of the STL, but for a given STL file it appears consistently even if I change print settings, including anti-aliasing, slicing settings etc.

I´m having the same problem, a plane parallel to the build plate that is not in the STL.

I am having a similar issue. I have a rectangular box that i hollow out with a smaller rectangle on the bottom, i insert a couple of mount pegs inside the hollow area (using Blender) and export as STL or OBJ. Windows preview shows correct geometry. Chitubox breaks the geometry. I have been fighting this for days. I can get around it sometimes, but am unsure of what i'm doing when it decides to randomly close the gap.