I think that if the OJP▸ doesn't provide a return fare, it is because it thinks the route is not valid as a through journey; it will offer the route as a combination of singles, but make it very hard to see which combination of singles it is offering.
For this interesting journey, OJP is not offering many valid journeys via any route; everything is via St Pancras but most suggestions are via Salisbury which apparently is not valid.
I think that's mixing up two distinct things. The OJP can cope with two singles as an alternative to a return; it has to for advance fares. That''s why it offers routes for both halves separately, but you choice of the outward fare alters what it offers on the return. In some ways doing research - looking at the offers without making a selection - makes it look confusing. For checking permitted routes and nothing else it's simpler to request a single, but obviously that tells you nothing about timing restrictions.
When it finds a journey (a sequence of trains) with a route it thinks is not permitted, it puts up that annoying "Multiple Tickets Required for Journey" flag. It doesn't tell you which options that applies to - sometimes there's no price, sometimes there's a price that looks like only part-route, and maybe sometimes it adds the part-route prices. The OJP is a weird and (sometimes) wonderful thing but I wouldn't accuse it of being consistently consistent.
But it's true this example is a complicated one. There are two valid routeing points at each end (so says the NRG's checker), and a total of twenty map combinations plus London routeing.
But to recap - before opening the NRG, any direct service is permitted (there are none), and any route with a lower specific fare (again, none as only Any Permitted fares exist). (You may recall that when discussing walking legs around Farnborough there were fares such as "via Farnborough North".) And the shortest route by track distance is also always permitted, but I wish you fun trying to find it! I guess it's a scenic route across southern England, with many changes, so a lot of work would be needed. That calculation cannot,
AFAIK▸ , include walking legs, or else I suspect that Tonbridge/Redhill/Guildford/Farnboroughs/Salisbury might be the one.
The routeing point for Warminster is Salisbury or Westbury, and in both cases the mapped routes are from London, one option from Waterloo and two from Paddington. Plus there's an oddity running from south wales to Waterloo through Warminster, but listed under Westbury. At the other end the routeing point is Strood or Tonbridge, and again all the options are to London - but there are more possibilities (2 and 4). None of them includes St Pancras, but that is covered under the London routeing. Finally, there can be a third map in the middle, always the one from Waterloo to Reading or Woking.
But with all these possibilities, during off-peak hours most journeys offered use
HS1▸ - quick and simple. Once timing restrictions kick in, more interesting journeys do come up - seven changes, for example. That's a result of the OJP trying very hard to find an off-peak option at 08:00, and succeeding. But it is odd that so many journeys offered don't match any of the permitted routes.
To return to the original question, forcing that walk at Maidstone gives only multi-ticket quotes, meaning the OJP thinks it's not permitted. But then it's not using either valid routeing point, so it wouldn't be, and I think that rule the OJP does follow.
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