It's not inherently problematic that such questions came from Stack Overflow originally. You may simply judge these questions as you would any other other self-answered question. Does the question deal with a social or legal question particularly applicable to FLOSS development or licensing? If yes, it belongs on this site. If you think it's a lazy question (or bad answer) you can downvote it as you like. If you think you have a better answer than the self-answer provided, you can give a different answer.
As for the self-answer originally being from another Stack Exchange user, this is indeed not socially ideal, but is not egregiously bad nor legally problematic (provided there is correct attribution). Ideally, the answer would belong, for purposes of site organization and reputation points, to the poster who originally made it on the other site. However, I think it's fair not to contact the answer's original author every time you want to transplant your question to another site, especially when the answer is already under a CC BY-SA license that allows you to do this freely. (I, for one, might even find these requests bothersome if I got them a lot.) Also, it may not be possible to contact the answer's author anymore for one reason or another. Stack Exchange's question migration solves all these problems, but the system has an arbitrary post-age limit so it cannot be used on old questions.
There are some adjacent concerns like
A user is flooding the site with a high volume of self-answered posts.
A user is reposting the entire GPL FAQ, item by item on the site. (In fact this happened at a small scale early in the site's history.)
The user is reposting other people's questions or answers without attribution.
But this case seems low-volume enough (and appropriately attributed) that these kinds of social/timing problems do not apply here. Ultimately, if the question is a good fit, it increases the volume and value of on-topic knowledge on the site.