logback elasticsearch appender missing index in Elasticsearch - spring-boot

Question

I have a Spring Boot application.
I want to send my logs directly from logback to elasticsearch. I'm using Logback Elasticsearch Appender (https://github.com/internetitem/logback-elasticsearch-appender)
The problem is that the index is not created in Elasticsearch. The File appender is working as expected and there are no errors in the logs or anything that shows a misconfiguration.
logback.xml:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
<file>
myApp.log
</file>
<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
<fileNamePattern>
myApp.%d{yyyy-MM-dd}.log
</fileNamePattern>
<maxHistory>
15
</maxHistory>
</rollingPolicy>
<encoder>
<pattern>
[%d] [%thread] [%-5level] [%class.%method] [user=%mdc{user:-n/d}] - %msg%n%ex{20}
</pattern>
</encoder>
</appender>
<appender name="ELASTIC" class="com.internetitem.logback.elasticsearch.ElasticsearchAppender">
<url>http://localhost:9200/_bulk</url>
<index>dev_myapp-%date{yyyy-MM-dd}</index>
<type>logback</type>
<connectTimeout>30000</connectTimeout> <!-- optional (in ms, default 30000) -->
<errorsToStderr>false</errorsToStderr> <!-- optional (default false) -->
<includeCallerData>false</includeCallerData> <!-- optional (default false) -->
<logsToStderr>false</logsToStderr> <!-- optional (default false) -->
<maxQueueSize>104857600</maxQueueSize> <!-- optional (default 104857600) -->
<maxRetries>3</maxRetries> <!-- optional (default 3) -->
<readTimeout>30000</readTimeout> <!-- optional (in ms, default 30000) -->
<sleepTime>250</sleepTime> <!-- optional (in ms, default 250) -->
<rawJsonMessage>false</rawJsonMessage> <!-- optional (default false) -->
<includeMdc>false</includeMdc> <!-- optional (default false) -->
<maxMessageSize>-1</maxMessageSize> <!-- optional (default -1 -->
<!-- <authentication class="com.internetitem.logback.elasticsearch.config.BasicAuthentication" /> optional -->
<properties>
<property>
<name>severity</name>
<value>%level</value>
</property>
<property>
<name>thread</name>
<value>%thread</value>
</property>
<property>
<name>stacktrace</name>
<value>%ex</value>
</property>
<property>
<name>logger</name>
<value>%logger{16}.%method</value>
</property>
<property>
<name>server-time</name>
<value>%date{ISO8601, America/Costa_Rica}</value>
</property>
</properties>
<headers>
<header>
<name>Content-Type</name>
<value>text/plain</value>
</header>
</headers>
</appender>
<root level="INFO">
<appender-ref ref="FILE" />
<appender-ref ref="ELASTIC" />
</root>
<logger name="com.myCompany.myApp" level="DEBUG" />
<logger name="org.hibernate.hql" level="OFF"/>
<logger name="com.zaxxer.hikari.HikariConfig" level="DEBUG"/>
<logger name="com.zaxxer.hikari.pool.ProxyLeakTask" level="DEBUG"/>
</configuration>
I'd expect to see dev_myapp-2022-01-20 in my local Elasticsearch index, but it's not there.
Elasticsearch is running locally:
curl localhost:9200
{
"name" : "ubuntu-dev-stage-services",
"cluster_name" : "elasticsearch",
"cluster_uuid" : "B0K_ThdDTlWftcVw23_NKA",
"version" : {
"number" : "7.16.3",
"build_flavor" : "default",
"build_type" : "deb",
"build_hash" : "4e6e4eab2297e949ec994e688dad46290d018022",
"build_date" : "2022-01-06T23:43:02.825887787Z",
"build_snapshot" : false,
"lucene_version" : "8.10.1",
"minimum_wire_compatibility_version" : "6.8.0",
"minimum_index_compatibility_version" : "6.0.0-beta1"
},
"tagline" : "You Know, for Search"
}
And this are the index in the instance:
curl localhost:9200/_cat/indices
green open .geoip_databases y1revQR7SyWHJIxnAq0z7w 1 0 42 0 40.3mb 40.3mb
green open .kibana_7.16.3_001 zjnnLKjXT362iQdFnXPsHQ 1 0 279 25 2.4mb 2.4mb
green open .apm-custom-link t8-PvRWaTpSd1n2xuT8tYA 1 0 0 0 226b 226b
green open .apm-agent-configuration hx8SsYzkTDqkEPpA7aZcvg 1 0 0 0 226b 226b
green open .kibana_task_manager_7.16.3_001 H-Kjao4hRLW45PmEMTvnGw 1 0 17 11520 1.6mb 1.6mb
green open .tasks Xp2ezFegRo-jzjK4UfUTpQ 1 0 6 0 28.9kb 28.9kb
EDIT
I was able to make it work by using Elasticsearch 5. I'm not sure but looks like the appender does not work with Elasticsearch 7