Annotation fails to anchor on pdf.

## Bug report form
### Steps to reproduce
1. http://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0168597&type=printable
2.Check orphans tab
3.This annotation failed to anchor on the first sentence of the abstract

### Expected behaviour
I made an annotation on the abstract of this article in the html version. https://hyp.is/lL9lhmfnEeeSdveBo58gHA/journals.plos.org/plosone/article?id=10.1371/journal.pone.0168597

All the other annotations synced themselves with the pdf, but this one shows up as an orphan. See figure.

### Actual behaviour
Tell us what happens instead.

### Browser/system information
Tell us which browser, browser version, and operating system you were using when
you encountered this issue.

### Additional details

## Feature Request Form
### Problem you are trying to address with this feature

### Your solution

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Reproduce the issue by opening the linked PLOS PDF, checking the orphans tab, and comparing it with the linked HTML annotation. Start by investigating why the annotation on the first abstract sentence does not anchor in the PDF; done means that annotation synchronizes with the PDF like the others.

Written by the indexing model from the issue text.