In your case LOG_PATH is not defined on startup. You should use ${LOG_PATH:-.} instead , See .
But if you define logging.path in your application.properties you will see two log files in . and in ${logging.path} directory.
Spring container set LOG_PATH after Logback initialization… Logback is not supported lazy file creation as far as I know. In this case you should use logback-spring.xml instead logback.xml.