I keep seeing references to sterile or nearly-sterile cultivars of butterfly bush. Having worked on eradication projects I'm still leery. Nearly sterile doesn't work for me. It's like almost pregnant. Additionally, having seen historical industry sterility claims regarding sterility in plants not only later be found to be not sterile but also quite invasive, I have not made the jump to butterfly bush. So, regarding butterfly bush, is there actually such a beast?

I don't have an answer to your question about a sterile type
but one can diligently dead head the flowers and thus no seeds
I don't grow them here , they don't come back , I do however have a couple clients that have bushes and I regularly dead head the flowers so there are no seeds.

I have two "Flower Power" Buddleia's to attract pollinators to my vegetable patch. I cannot guarantee they are 100% sterile but they haven't propagated yet and seem to be the choice for non-invasive Buddleia's here in Europe.
The only thing I can say is they are among the most effective Buddleia's I have seen when it comes at attracting pollinators... the amount of butterflies, bumblebees etc they attract can get scary.