Generally, if you can provide a source to show notability for the specific 'There has never been' / 'Has there ever been' claim, I'd say it is on topic (and we do have a couple of well received has there ever been questions).
And generally, you don't have to search for the original claim if you can provide a notable source which repeats the claim. I'd say that holds true even if it is repeated in a relatively weak 'some people claim' form. You shouldn't build your own claim though (so if you have a source that claims X about trans people, but doesn't mention Chelsea Manning, I don't see a reason to include her in the question).
But with this specific example, I'd still consider it off-topic because it's about Motivation. We don't accept questions which ask why a person did a thing, because it's not answerable using verifiable facts.
Apart from that, I'd also be worried about the question pushing a POV (which doesn't necessarily make a question off-topic here, but which might attract downvotes).
Even if there is a case of this happening (which seems doubtful - after all, why go through the lengthy and difficult process of transition, if you could just as well adopt a different persona without going through transition -), what does that mean? That we shouldn't allow trans people to transition? That we should treat trans people who do transition differently? We don't draw those conclusions for cis people (who also do crime), so it seems like a double standard.