Why I see "null" statements in between logs (With Filter class extended from OncePerRequestFilter)

Question

I have added a Filter class which extends OncePerRequestFilter and whenever API's get called along with normal log statements I see null statements.

Sharing logback.xml configuration and Request Filter code is straight forward which logs collect some informations and log the statements.

Why this happens any help will be appreciated.

{"@timestamp":"2022-09-15T18:21:19.295+05:30","@version":"1","message":"Started Application in 20.694 seconds (JVM running for 21.915)","logger_name":"logger-name","thread_name":"main","level":"INFO","level_value":20000,"app":"app-name"}
null
null
null
null
null
null
null
null
null
null
null
{"@timestamp":"2022-09-15T18:21:31.655+05:30","@version":"1","message":"Received request from app swagger-ui","logger_name":"logger_name","thread_name":"http-nio-8080-exec-1","level":"INFO","level_value":20000,"clientIp":"0:0:0:0:0:0:0:1","serviceName":"client-name","transactionId":"CBC76C0E4A8B4C4798599AEC3C91DD3B","app":"app-name"}

Logback xml file

<configuration>

<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="net.logstash.logback.encoder.LogstashEncoder">
        <customFields>{"app":"app-name"}</customFields>
    </encoder>
</appender>

<appender name="appLogAppender" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
        <providers>
            <timestamp/>
            <threadName/>
            <provider class="net.logstash.logback.composite.loggingevent.ArgumentsJsonProvider"/>
        </providers>
    </encoder>
</appender>

<logger name="console-logger" level="INFO" additivity="false">
    <appender-ref ref="CONSOLE"/>
</logger>

<logger name="applicationLog" level="INFO" additivity="false">
    <appender-ref ref="appLogAppender"/>
</logger>

<root level="ERROR">
    <appender-ref ref="CONSOLE"/>
    <appender-ref ref="appLogAppender"/>
</root>