ElasticSearch (6.5) Java high level rest client - must(QueryBuilders.rangeQuery) and mustNot(QueryBuilders.existsQuery) cmbination not working

Could you provide a full recreation script as described in About the Elasticsearch category. It will help to better understand what you are doing. Please, try to keep the example as simple as possible.

A full reproduction script will help readers to understand, reproduce and if needed fix your problem. It will also most likely help to get a faster answer.

Here I'm not sure on how you are calling the search endpoint.
Having a full script that reproduces the problem with Kibana dev console would help a lot to understand what you are doing wrong.