You can no longer access FRawCurveTracks due to recent changes in the engine. It will work during development but packaging a shipping build causes animations to be cooked with different compressions (which you can choose) and the raw track data is no longer available.
There is a new compression codec upcoming in UE 4.24 that will preserve this data. You can either wait it out till the next release or you can store your distance curve data externally from the animation.