handleConextUpdate never handles insert requests but only delete request. Bug in GraphDB plugin API

Question

if I register a class as a Plugin service which implements the ContextUpdateHandler and overrides the getUpdateContexts and handleContextUpdate methods like below, SPARQL insert requests will never be handled. Might there be a bug in the Plugin API implementation?

@Override
public Resource[] getUpdateContexts() {
    getLogger().info("getUpdateContexts");
    Resource[] res = new Resource[1];

    res[0] = () -> "";

    return res;
}

@Override
public void handleContextUpdate(Resource subject, IRI predicate, Value object, Resource context, boolean isAdded, PluginConnection pluginConnection) {
    if (isAdded)
        getLogger().info("Handle insert request"); //never reached
    else {
        getLogger().info("Handle delete request"); //is printed. works fine

    }
}

Update statements

insert data {<http://example.com/test/s/insertThis1> <http://example.com/test/p/insertThis1> <http://example.com/test/o/insertThis1}

delete data {<http://example.com/test/s/insertThis1> <http://example.com/test/p/insertThis1> <http://example.com/test/o/insertThis1}