year == 201[8-9] returns nothing, but year==2018 and year == 2019 do.
Was there any change in the search implementation which was not added into the help, or could it be a bug in my current version?
Does it work for you in a newer jabref version?
JabRef 5.2–2020-12-24–6a2a512
Linux 5.11.5-051105-generic amd64
Java 15.0.1