Q:

logback file disappears after a few days running

I have a java program named appname, it's using logback and sl4fj for logging.
Expectations:

The log file is rolling every day at midnight and all the generated logfiles will be kept for 30 days based on 
the settings <maxHistory>30</maxHistory>
Used absolute path to specify log file pattern  like <fileNamePattern>/var/log/appname/logfile.%d{yyyy-MM-dd}.log</fileNamePattern>
Created a user appname on linux server(CentOS) to be the owner of the java program appname.jar and
/var/log/appname, only this appname user is allowed to run the program.

Problem I encounter:
I leave it running, unfortunately 5 days after the appname program starts up, the files rollings work fine BUT the logfile.log(rolling for logging current day's data) disappears,
all the generated log files (previous days logs) are still there. 
Edit: I ran the program two times, the logfile.log is gone on the 4th day(the 1st time I ran it), and is gone on the 5th day(the 2nd time I ran it) 
HELP:
Does anyone have any idea about this? 
My Guessing:

Is it related with logback&sl4fj 
Permission issue? 
Is it beacuse of the log file size(did not set the limit size, it is around 15GB for each)?

    <configuration debug="true">
        <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
            <encoder><pattern>%-4relative [%thread] %-5level %logger{35} - %msg %n</pattern></encoder>
        </appender>
        <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
            <file>/var/log/appname/logfile.log</file>
            <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
                <fileNamePattern>/var/log/appname/logfile.%d{yyyy-MM-dd}.log</fileNamePattern>
                <maxHistory>30</maxHistory>
            </rollingPolicy>
            <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
                <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5p %c - %m%n</pattern>
            </encoder>
        </appender>
        <root level="DEBUG">
            <appender-ref ref="FILE" />
            <!-- <appender-ref ref="STDOUT" /> -->
        </root>
    </configuration>
    <logger name="org.apache.zookeeper">
        <level value="ERROR"/>
        <appender-ref ref="FILE"/>
    </logger>

Any help will be appreciated, thanks in advance!

A:

<?xml version="1.0" encoding="UTF-8"?>

with multiple days testing, I found that the above line was missing in my logback file, after I added it as the first line of my logback file, the rolling works properly, hope it can help people have the similar issue with me.

