The only fares from Langwathby to Longbeck are an Anytime Day Single and Anytime Return, both routed "via Hexham". This is valid on all operators along its permitted routes, which include the ECML between Newcastle and Darlington.
If you search for Langwathby to Longbeck specifically (and enter appropriate via/TOC restrictions), TrainSplit will happily offer you an itinerary which involves LNER on that stretch:
TrainSplit compares routes & fares to get cheap train tickets for your next journey through split ticketing with no booking fee & saving up to 90%
trainsplit.com
However if you search for Skipton to Longbeck and enter equivalent restrictions to force an itinerary via Carlisle, it only offers splits at Hellifield, Newcastle and Darlington as you say:
TrainSplit compares routes & fares to get cheap train tickets for your next journey through split ticketing with no booking fee & saving up to 90%
trainsplit.com
The most likely reason I can think for this is that retailers like TrainSplit are restricted in terms of the number of queries they're allowed to make to RARS (the industry reservations system). In particular, they have to keep their 'look to book' ratio (i.e. the number of search queries vs bookings) within a range that RDG (the industry central body) find acceptable. Otherwise they could get blocked/restricted from RARS, or be charged additional fees that would likely make their business model unviable.
The reason this matters, even when selling a walk-up ticket, is that LNER have added a fake 'reservations compulsory' flag to all of their trains. This means that retailers can only offer an itinerary involving LNER if they have checked in RARS to see that there are seats available on the LNER train. Unfortunately for split ticket retailers, that availability check has to be done individually for each split combination - i.e. it can't simply be done for the LNER leg itself (Newcastle to Darlington).
herefore, when there are lots of possible split options, only some of them can actually be checked for itineraries that involve 'reservations compulsory' services like LNER's. I believe a certain amount of prioritisation of specific split stations (especially larger ones like Newcastle and Darlington) takes place on TrainSplit when this is the case. By contrast, TPE and XC don't have the same fake 'compulsory' flag (on that route/date, at least) so TrainSplit's data provider can perform an exhaustive search to find the best possible split point, which is Langwathby rather than multiple splits.
So basically - TrainSplit isn't offering you the optimal split, but I believe the reason is down to industry limitations.
