Label for rule is not applied by method, but works with annotation

When attaching a label to a Rule via method .label("some label") it is not correctly applied to the rule. Attaching a label via annotation @Label works fine.

Example:

import org.parboiled.Parboiled;
import org.parboiled.Rule;
import org.parboiled.annotations.Label;
import org.parboiled.parserunners.TracingParseRunner;
import org.parboiled.support.ParsingResult;

import com.github.parboiled1.grappa.parsers.EventBusParser;

public class LabelTest {

    public static void main(String[] args) {
        LabelParser parser = Parboiled.createParser(LabelParser.class);
        for (Rule r : new Rule[] { parser.rule1(), parser.rule2() }) {
            TracingParseRunner<String> runner = new TracingParseRunner<>(r);
            ParsingResult<String> result = runner.run(new StringBuffer("a"));
        }
    }

    public static class LabelParser extends EventBusParser<String> {

        @Label("Label-1")
        Rule rule1() {
            return optional("a");
        }

        Rule rule2() {
            return optional("a").label("Label-2");
        }

    }

}

Output:

Starting new parsing run
Label-1/'a', matched, cursor at 1:2 after "a"
Label-1, matched, cursor at 1:2 after "a"
Starting new parsing run
rule2/'a', matched, cursor at 1:2 after "a"
rule2, matched, cursor at 1:2 after "a"

The label "Label-1" of the first rule is applied correctly, the second label "Label-2" is not applied. The second rule is still labeled as "rule2" in the output. This also happens in the tracer / debugger, so it is not a problem during output, but when attaching the label.