Looks like this is in bolognese not lupo and how we handle date parsing in general.
I've created a branch with a rspec test that causes the issue:
https://github.com/datacite/bolognese/tree/fractional_date_bug_1361

Run the specific test
bundle exec rspec ./spec/readers/datacite_reader_spec.rb:1656

The bug or arguably just how it's implemented is down to our use of a EDTF library for date parsing
https://github.com/datacite/bolognese/blob/6cb2ae3d76eca7e5e88142598e6b331850342673/lib/bolognese/readers/datacite_reader.rb#L128

But reading the datacite schema spec, fractional dates are allowed and therefore EDTF doesn't quite cover this use case.

I suspect we went with EDTF back in the day because it was closest to what we needed.

If you look at the XSD https://schema.datacite.org/meta/kernel-4.4/metadata.xsd the descriptive text has:
YYYY,YYYY-MM-DD, YYYY-MM-DDThh:mm:ssTZD or any other format or level of granularity described in W3CDTF. Use RKMS-ISO8601 standard for depicting date ranges.

So need a library that conforms to W3CDTF but I believe ISO 8601 is part of this.

If we change this from EDFT then I suspect we need to change every other reader not just datacite_reader, it will also notably break a few more specs when the change is made.