roate_size not working as expected #4507

<system>
  <log>
    rotate_age 7
    rotate_size 8192
  </log>
</system>
<source>
  @type tail
  path /var/log/pods/*/*/*.log
  exclude_path ["/var/log/pods/*/*fluentd*/*.log"]
  pos_file /var/log/td-agent/container.log.pos
  tag omera.*
<parse>
    @type none
</parse>
</source>
 
<filter names.**>
  @type record_transformer
  enable_ruby
<record>
    file_name ${tag.split('.')[5]}
</record>
</filter>
 
<match fluentd.**>
  @type null
</match>
 
<match names.**>
  @type file
  append true
  path /fluentd/log/${file_name}.%Y%m%d.log
<buffer file_name,time>
    timekey 1d
    timekey_use_utc true
    timekey_wait 5s
    chunk_limit_size 8192
    flush_at_shutdown true
    flush_interval 1s
    flush_mode interval
</buffer>
<format>
    @type out_file
    time_slice_format %Y%m%d
    time_format %Y-%m-%dT%H:%M:%S%z
</format>
</match>