I’d also guess, without much evidence, that there’s a halo effect-like thing going on where if someone really care about averting animal suffering a vegan diet starts seeming more virtuous, which spills over into their assessment of its health benefits.
I think your confounders are on the money.
You might be interested in Elizabeth’s Change my mind: Veganism entails trade-offs, and health is one of the axes. I especially appreciated her long list of cruxes, the pointer to Faunalytics’ study of nutritional issues in ex-vegans & ex-vegetarians, and her analysis of that study attempting to adjust for its limitations which basically strengthens its findings (to my reading).
I’d also guess, without much evidence, that there’s a halo effect-like thing going on where if someone really care about averting animal suffering a vegan diet starts seeming more virtuous, which spills over into their assessment of its health benefits.