My Autumn planted ones have bolted quite a lot, maybe 50%, but a good crop nevertheless. They are rather large.
I suspect more and more onion sets are not treated in the way they should be, especially Autumn planted ones. I have been swapping to seeds for the last 3 years and have quite a few in the ground this year. Still learning about onion seeds, not the easiest to grow given their timings, but just about there now. The increased possibility of importing white rot on sets is another reason I have been swapping to seed.