As discussed on
DS-2699, end users appear generally to expect search to "do what Google does". This includes the expectation that searching for multiple words will do an AND search, that is, the results list will be those items that contain all of these words (rather than an OR search: any of these words).
The default DSpace search operator has been OR for a long time. There is a configuration setting in dspace.cfg to change the default operator, but the setting is not honoured by Discovery. Instead, the setting needs to be made in the schema.xml file for the discovery solr core. As well as changing the default operator, the documentation will also need to be amended so that sites wishing to use OR as the default know where to change this.