You’re right, that’s a bug!
Use this instead if you can compile code:
// We can't just project Delta onto the plane defined by FloorNormal because the direction changes on spherical geometry
FVector NewDelta = FQuat(DeltaPlaneNormal ^ DeltaNormal,
(((FloorNormal | Delta) > 0.0f) ? 1.0f : -1.0f) * FMath::Acos(FloorNormal | DeltaPlaneNormal)).RotateVector(Delta);