Weird behaviour on document update to remove value from a field

Elastic Stack Elasticsearch
10

Unless I am making a mistake in the below code.
I would say I am not using any custom configuration.

builder = RestClient
                .builder(new HttpHost(this.targetUrl.getHost(), this.targetUrl.getPort(), this.targetUrl.getScheme()));


        RestClient lowLevelRestClient = builder.build();
        transport = new RestClientTransport(
                lowLevelRestClient,
                new JacksonJsonpMapper()
        );
        clientES = new ElasticsearchClient(transport);