A possible way to demonstrate the problem is to export G4Para to gdml file when internally G4UPara is used (probably when compiled with VecGeom), e.g. /cvmfs/sft.cern.ch/lcg/views/LCG_101. The X size of the G4Para exported to gdml via G4UPara equals to its Z size: G4double G4UPara::GetXHalfLength() const { return GetZ(); }
Thanks for the report. The fix is now submitted to the development master and will be included in next release.