I don't think you mean "valid" here; you mean "the same as", I expect? What you are describing are fare clusters, and it is indeed the use of a broad fare cluster for a relatively short journey that causes the problem.
As father_jack says, the problem was caused by the industry's ongoing efforts to remove "any permitted" tickets. The fare from "Bristol cluster" to Cardiff Central (Q066-Q496) had an any permitted route until fairly recently, possibly September. The fare from Bristol Parkway to Cardiff Central (3230-3899) was also "any permitted" and overrode it because that's how fares work.
Now the "Bristol cluster" fare is "via Newport", and the Bristol Parkway one "any permitted", it doesn't override it, and both tickets are available to buy. I would hope that retail systems would strongly promote the cheaper fare, but people may end up with the wrong ticket anyway.
For the other stations in the cluster:
Patchway-Cardiff Central has a "via Newport" route, so it does override the "cluster" version.
Filton Abbey Wood-Cardiff Central is the same.
Pilning-Cardiff Central has an 01000 route, so both versions of the fare are available.
None of the other stations have a separate set of fares to Cardiff Central.
In an ideal world, the people who make changes like this would have a tool which let them know if a change of route code meant that a flow had stopped overriding another one (or, indeed, if it had started overriding something) before it went into use. It doesn't appear that anyone uses something like this, though.