I don't see how anyone who reads "WMT & NTH ONLY" or "Valid only on West Midlands Trains and Northern services." would think their ticket is valid on Transport for Wales.
Well I thought it was valid on TfW - and with very good reason! I'd asked at the booking office for a LNWR ticket from London to Manchester - a ticket that for as long as I can remember, included TfW. I got sold a ticket which was exactly the price I expected to pay for it, and on a very quick inspection as soon as I was sold it, said London to Manchester super-offpeak. On all previous occasions when I'd checked the fares, there was no comparable ticket that wasn't valid on TfW (because it seems the LNWR/Northern ticket is a very recent thing and presumably didn't exist when I'd previously checked the fares). So without a very detailed look at the ticket, there was simply no reason to expect it not to be valid on TfW. Even when later on I did notice it said 'Northern', although I was a bit puzzled about it, my best-guess assumption was that perhaps that referred to the 'North' part of the LNWR name, or was some weirdness in how the ticket was printed - which wouldn't be totally unheard of. It was only really once I was back home after completing my return journey that I thought about it more, became more puzzled, and decided it was worth asking here.
Now consider that I'm far more knowledgeable than most members of the public about train services and ticketing, and imagine how it would be for a more typical member of the public who buys an 'LNWR' ticket and perhaps isn't really even aware of the names of the TOCs but has simply heard that the LNWR tickets are the cheapest...
So yes, I would say it's perfectly possible for someone to legitimately genuinely believe that ticket is valid on TfW. Having two tickets at exactly the same price, with the same main TOC for the bulk of the journey, but different secondary TOCs, and with one of the tickets having only recently come into existence so many people won't be aware of it even if they know about the other ticket, is just asking for this kind of confusion.
If the passenger didn't actually look to see what ticket they had purchased, I don't see how that is anyone's fault except the passenger.
So you expect every rail passenger to carefully look at and understand the detail of every bit of information that's printed on their ticket (not just the obvious information like the origin, destination and ticket type)? Maybe in your World, noone who isn't a complete expert in railway ticketing should be allowed to travel by train?