I've come across a few of these recently and just worked out the difference (or at least one of them) - if the "Scroll to Text Fragment" ends with a period it fails to fill out
There may be other cases as well, but hopefully looking at this case will identify any underlying issue.

Note "Scroll to Text Fragment" currently only works on Chrome - see https://chromestatus.com/feature/4733392803332096

Steps to Reproduce:
Add these refs to a page and run refill - the first with no period on the end works (as long a we are happy with removing the text fragment) the others fails

<ref>https://www.medicalnewstoday.com/articles/322666#:~:text=Ovarian%20torsion%20is%20a%20condition,is%20not%20receiving%20enough%20blood</ref>
<ref>https://www.medicalnewstoday.com/articles/322666#:~:text=Ovarian%20torsion%20is%20a%20condition,is%20not%20receiving%20enough%20blood.</ref>