The java-logging library updated its flushlevel default to OFF in Dec 2018 due to the performance cost of flushing on the same thread as the application.

However, the java-logging-logback appender was implemented in May 2017 to default to ERROR, and continues to default to ERROR.

( Add'l context: This came to our attention with a request to make Spring Cloud GCP's helper-implementation of a logback appender match the base library's default. See GoogleCloudPlatform/spring-cloud-gcp#2906 for discussion. )