Ansys Employee

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.