I think I can reproduce this one too.
Route '.' (Any Permitted) Super Off-Peak CBA fare set by Thameslink/Great Nothern...
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Exactly the same train for the same segment, more expensive Off-Peak, route '.' (Any Permitted) fare... magically, it's got availability!
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And of course, GR144700 (like all LNER services) has the "Compulsory Reservations" flag set so this is more than just denying passengers seats, it denies retailers from selling completely valid itineraries against these tickets.
How many passengers have been overcharged due to this sort of nonsense?