"citizen" is pretty well defined in every country i've ever been in.

Maybe legally, but that's not what I am referring to.

They downvote because it makes sense and they can't refute it

How is the argument much different than any current arguments? You can already get significant benefits from the state as a citizen (in European countries, at least).

The difference is magnitude, the benefits that European countries provide has made them a very attractive destination for immigration, so it stands to reason that something like UBI would make them even more so. No matter which side of that debate you're on theres a point where the math breaks down and some difficult choices need to be made, either you provide these generous benefits to your citizens or you have a generous immigration policy, but both of them together may prove unsustainable

Which is why unchecked immigration is already a problem. Increasing the benefits without dealing with unchecked immigration is going to make things worse.