Annotations don't anchor in iframe if fetched based on Dublin Core identifier

Minimum Example: I have two file `test1.html` and `test2.html` each contain an iframe using srcdoc or src with
```

```
Highlight/annotation inside one iframe become orphan for another iframe.
I suspect the cause is that when receiving annotation from server, the `target` property of annoation is usually url rather than fingerprint-like urn:x-dc:....

Note that even if i am not using srcdoc, using src with **even the same link** still cause similar phenomenon; also setting `test1.html` and `test2.html` with the same dublin meta data can't fix the problem.