GeoNetwork is a catalog application to manage spatially referenced resources. It provides powerful metadata editing and search functions as well as an interactive web map viewer. It is currently used in numerous Spatial Data Infrastructure initiatives across the world.
I think keyword are indexed with a KeywordAnalyzer so it is case sensitive due to that. An option would be to add another field with a StandardAnalyzer
see
https://vanilla.geocat.net/geonetwork/srv/dut/csw?request=GetRecords&service=CSW&version=2.0.2&elementsetname=full&typenames=gmd%3AMD_Metadata&RESULTTYPE=results&constraintLanguage=CQL_TEXT&constraint_language_version=1.1.0&maxRecords=100&startPosition=1&constraint=%28%28Subject+LIKE+%27%25boundaries%25%27%29%29
vs
https://vanilla.geocat.net/geonetwork/srv/dut/csw?request=GetRecords&service=CSW&version=2.0.2&elementsetname=full&typenames=gmd%3AMD_Metadata&RESULTTYPE=results&constraintLanguage=CQL_TEXT&constraint_language_version=1.1.0&maxRecords=100&startPosition=1&constraint=%28%28Subject+LIKE+%27%25Boundaries%25%27%29%29
users don't expect this type of fields to be case sensitive