As you're calculating the permitted route to a zonal destination (LU Zones 4-6), you take the first station within the zonal validity the train travels through (doesn't need to stop) as the intended destination for routeing. This would be Hatch End.
If you use the routeing point calculator, this shows that there were no NFM64 fares defined between Elstree and Hatch End, meaning that no mapped journeys are available. This leads only the direct train route and the shortest +3mi rule.
The shortest journey rule includes
only rail legs, so any fixed links, buses, etc are ignored.
In this case, the route via West Hampstead is 31.59 miles, but the journey to St Albans City, then Abbey to Hatch End totals to around 18 miles, making it clearly the shortest route, as a result TrainSplit provides it as a valid journey itinerary as it should, including on point to point journeys:
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