Incorrect face index from trace

[Pacinho]

I'm trying to get triangle index through face index from trace and returned face index is inconsistent. It happens with BasicUsageActor example, as with my own mesh. It's either correct one or index that is higher than total triangles in mesh.
The difference between indices is same across all triangles and the more triangles there are, the bigger difference, with difference/total triangles ratio never reaching 10(e.g BasicUsageActor has 36 total triangles, difference is 72; with 131232 triangles difference is 1312200).