You have found a strange behavior. I made a quick test and it worked, but then I made another one and it doesn't.
So we have to inspect this. Unfortunately half of Germany enjoys today a day off and on Friday a bridge day, this includes our developers 🤷♂️ If possible I'd like to move this issue to Monday. If not, I have to wake up a developer 😆
In any case I would recommend that you give in the target document an anchor. This makes it more robust when you change the headline afterwards:
[#my_anchor]
== My headline
In the referencing document you enter:
Click for xref:/Document.adoc#my_anchor[more information].
Thanks. I'll keep you posted.