When building a query e.g. for a boolean property:

QueryBuilder qb = ...;
kb.keyword()
    .onField( "active" )
    .matching( true )
    .createQuery();

Then we pass the String "true"/"false" as part of the JSON query instead of the actual boolean.

It doesn't make a difference in this specific case as ES will still return the right results by converting the value itself, but it might not be working for some other cases (dates?).