Is it possible that the bun was both plant-based and gluten-free?
The impression I get from reading and watching restaurant reviews is that Disney occasionally tries to kill two birds with one stone by providing one product for both dietary needs. It makes sense, because it would cut down on costs to have two bun options (regular and allergy-friendly) rather than three (regular, plant-based, and gluten-free), but plant-based eaters are likely to lose out when that's the case. Vegan bread is easy to make yummy. Gluten-free bread is harder. Combine the two and it's even more of a challenge.
I don't have any specific insight about Casey's, so I have no idea if that's the case there. Does it seem like a possibility?