In 3D PDF, there is a concept called “Crease Angle”. Crease Angle effects how angular features are presented, lower values will provide better representation whilst higher values will see less representation.
The effect of this feature is that for surfaces with curvature below this angle, or adjacent faces sharing an edge, the display assumes these are smooth, and displays them in a way they may not appear flat.
To control this in ReportGen, go the "Visual Effects tab. Near the bottom there is a control value for “Crease Angle”. The default value is 45 degrees, lowering to 10 degrees will make more surfaces be considered flat rather than smoothly curved.