Interior threads don't always have a normal so unless you're reporting the cells for c0 and c1 from the same facet you could see the above. I think there's a comment in UDF manual somewhere, it is/was mentioned in the surface reports in Fluent because it can mess with the post processing reports.
The UDF manual is a good source of information. We do provide training too, but you'd need to request that (or much cheaper, Ansys Learning Hub access) via your University.
You are navigating away from the AIS Discovery experience