If an engineering candidate's last few positions have all been about a 1-year vest, that's definitely going to be on my mind. I'd prefer to build a coherent, highly-effective development team who can see us through the whole lifecycles of this and successive projects -- not what might as well be fly-by-night gig contractors, tossing less-aligned check-off sprint deliverables over the wall.
I don't currently have a standard way to ask about this, and would play it by ear. Maybe their answer is that there was something about recent positions that they didn't like, or maybe they've been doing early startups, or maybe they were junior but just now hitting their stride as experienced and know what they want and how to perform, or maybe they give what seems like a dishonest answer, or maybe they're only looking for particular kind of short-term transactional engagements (which we might need).