Looking at this DOI: 10.1175/1520-0477(1996)077<0935:WOTWSM>2.0.CO;2 (similar behavior observed with other "weird" DOIs)
We observer the following behavior:

1. https://doi.org/10.1175/1520-0477(1996)077<0935:WOTWSM>2.0.CO;2
   resolves without issues
2. the CrossRef API returns metadata for both the regular and the URI encoded version of the DOI, i.e. https://api.crossref.org/works/10.1175/1520-0477(1996)077<0935:WOTWSM>2.0.CO;2
   works, as does https://api.crossref.org/works/10.1175%2F1520-0477%281996%29077%3C0935%3AWOTWSM%3E2.0.CO%3B2
3. Content negotiation works when requesting bibtex: curl -LH "Accept:application/x-bibtex" https://doi.org/10.1175%2F1520-0477%281996%29077%3C0935%3AWOTWSM%3E2.0.CO%3B2
   So far so good! BUT
4. Content negotiation requesting CrossRef Unixref silently fails: curl -LH "Accept:application/vnd.crossref.unixref+xml" https://doi.org/10.1175%2F1520-0477%281996%29077%3C0935%3AWOTWSM%3E2.0.CO%3B2 returns empty.
5. As an additional clue, search.crossref.org also comes up empty: https://search.crossref.org/?q=10.1175%2F1520-0477%281996%29077%3C0935%3AWOTWSM%3E2.0.CO%3B2

I know these are old and relatively rare DOIs, but we've already gotten the first complaint 5 days after switching Zotero's DOI import to content negotiation, so would love to get this working.

CC @zuphilip