SBend3D field map description does not mention the mirror symmetry about the midplane.
As discovered by @nesteruk_k, the SectorMagneticFieldMap
class applies mirror symmetry around the midplane: https://gitlab.psi.ch/OPAL/src/blob/master/src/Classic/Fields/SectorMagneticFieldMap.cpp#L211
This should be mentioned in the manual, https://gitlab.psi.ch/OPAL/Manual-2.1/wikis/elements#sec.elements.SBend3D