There's a small number of ticket types with special refund conditions such as this (such as Caledonian Sleeper tickets).
These aren't present in any kind of data feeds, so it isn't really possible for us to provide accurate information for them unless we manually curate it. And we sell so few (often none?) so it doesn't make it worthwhile. In these cases, refunds would be on a manual and case-by-case basis rather than using our online portal.
This one is particularly bad because it's also shown as booked train only, while it's actually valid for any service on the day. And you get a 1st class upgrade. And if no 1st seats are available, you get £20 back.
It's probably why we seem to be the only ones that sell this ticket online...