This question has already been asked here, but it lacks some authoritative sources there. I once stumbled upon a source saying that in case with a territory it is correct to say "на" ("на Руси", "на Кубани", "на Урале"), and in case with a separate state, it is correct to say "в" ("в Японии", "в Ираке", "в России", "в Италии") with some exceptions of some island states when saying "на" can also be okay ("на Кипре", "на Ямайке", "на Мальте"). I can't find that source now, but I suspect that it was written long before this war in Ukraine, and even long before Ukraine became a separate state. And it looks like saying "на Украине", though quite against the above-mentioned rule, still seems to be okay - as a kind of exception to the rule.
Can anyone cite an authoritative source specifically addressing this case of "на Украине vs. в Украине"?