Game Build Linker Error Vanilla C++ Class

Development Pipeline & Plugins
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1

Just upgraded to Unreal 4.15, migrated my project. I can build fine using “Debug Editor”, “Development Editor”, etc.
But the moment I switch to “Development” or “Debug” for a non-editor build I get errors like:

2>FlightModelController.cpp.obj : error LNK2019: unresolved external symbol "public: static float __cdecl FlightMath::CalculateSlewAngle(struct FVector const &,float,float)" (?CalculateSlewAngle@FlightMath@@SAMAEBUFVector@@MM@Z) referenced in function "public: void __cdecl UFlightModelController::Tick(float)" (?Tick@UFlightModelController@@QEAAXM@Z)

Anything in FlightMath.h has linking errors, but works fine with Editor builds.

FlightMath.h is defined as:

#pragma once
#include "FlightStructures.h"

class FLIGHTMODEL_API FlightMath
{
public:
    static void CalculateComputedValues(FAircraftComponent& component);
    static float CalculateLift(float cl, float area, float velocity, float rho);
    ...

I’m using Visual Studio 2015.