Is Virgin ticketing one of those shoddy providers, like Uber and Trainpal, that is really rubbish, but people put up with all sorts of problems to get cashback and/or money off vouchers?
The first ticket is absolutely valid to St Pancras; the second ticket is superflous.
I don't know who does the stuff behind the scenes for Virgin ticketing, but it's not the same supplier the forum's site uses.
My advice would be to use the forum's site in future; bugs of this nature are very rare, and can be reported directly to the relevant people who can get any such issues fixed, and a refund for any missold ticket would likely be very prompt indeed.
AFAIK Virgin Trains Ticketing heavily partnered with Siverrail to the point where Silverrail do just about everything, basically a white label Silverrail app with added Virgin Red points. I've generally found the split ticketing functionality rather limited but there are no fees and it can often coaxed into using a different split point if you manually identify a better split (I did this to Ramsgate, it was wanting to split in London, quite possibly in such a suboptimal way as the OP had it do, yet I knew a cheap advance was available from Donny to Ramsgate, so entering 'Doncaster' in the via field made it split there. But I digress...)
Even appears on my credit card statement as 'Silverrail*VirginTrains' or something like that until the payment has fully cleared. I've actually found their support not bad, though as others as well as myself have discovered, the app has a few bugs. One, which I think (though can't be sure) has since been squished is that when booking an SVR or SOR, say, and selecting a return date, the 'start from' date was shown as the date selected rather than the actual beginning of validity. However I believe some other Silverrail apps used to have this issue?
I find that Virgin Trains Ticketing , along with the forums site and brfares.com are between them the third party retailers I'd generally recommend, each having a different usage case. I use VTT for my 'everyday' use, simply because of the Virgin Red points. They'd have added up a lot quicker though if I still commuted by rail on CDRs but I've nearly got a cheap flight to Amsterdam...
To be fair, yes I put up with the odd bug here and there to get points, but it's no worse than the bugs and design flaws I've found in some TOC apps. I've yet to find a perfect app/booking site for all use cases.