"sort by year" makes papers vanish

### Confirm that this is a bug report

- [X] I want to report an issue that does _not_ concern paper or author metadata.
- [X] I have searched for similar existing issues first.

### Problem Description

The following query gets "about 28" hits, including many paper PDFs. But when I select "Sort by Year of Publication," only 1 hit remains. Maybe this means that publication year is missing for many papers? (Should they be shown anyway, perhaps at the end of the listing?)

https://aclanthology.org/search/?q=snowdon+nun

> I want to report an issue that does not concern paper or author metadata.

Ok, it does concern those things, but not for a _specific_ paper ...

I'm using Google Chrome 114.0.5735.198 on Linux Mint.


Start by reproducing the behavior at https://aclanthology.org/search/?q=snowdon+nun, comparing the default results with the "Sort by Year of Publication" view. Trace the search and sorting entry points from that page; done means matching papers do not disappear merely because publication year is missing, with the handling of undated papers matching the agreed behavior.

Written by the indexing model from the issue text.