Wikipedia lists some limitations of the Wiedemann–Franz law[1], and also some previous violations in other materials.

Reading the Wikipedia page I don't get the sense the law is quite as fundamental as the headline and summary make it sound like.

Here's one of the previous violations:

In 2011, N. Wakeham et al. found that the ratio of the thermal and electrical Hall conductivities in the metallic phase of quasi-one-dimensional lithium molybdenum purple bronze Li0.9Mo6O17 diverges with decreasing temperature, reaching a value five orders of magnitude larger than that found in conventional metals obeying the Wiedemann–Franz law. This due to spin-charge separation and it behaving as a Luttinger liquid.

Still, graphene is cool and seems to be the gift that keeps on giving in terms of surprising results in solid state physics.

[1]: https://en.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law#Li...