Logstash Aggregate Filter output different result each time

I have 2 input files like below -
inbound.txt:
17:03:56.662 53897 55931
17:03:56.737 53898 55932
17:03:56.814 53899 55933
17:03:56.889 53900 55934
outbound.txt:
17:03:56.887 77307 55931
17:03:56.953 77308 55932
17:03:57.028 77309 55933
17:03:57.105 77310 55934
17:03:57.180 77311 55935

My aggregate filter part in Logstash config is like below. Purpose is to merge the lines with same 3rd ID ( I named it FIXinID ) into one events. After that I can calculate the time gab for each InID. My current trouble is, the aggregate filter sometimes worked perfect, sometimes not. I don't know how to finger it out. Can anyone help advise please?

filter {
    if [type] == "inbound" {
          grok {match => {"message" =>"%{TIME: TIME1}\s+.*\s+%{NOTSPACE: FIXinID}"}}
     }

   if [type] == "outbound" {
          grok {match => {"message" =>" %{TIME: TIME6}  {%NOTSPACE: OutID}  %{NOTSPACE: FIXinID} " }}
     }

   mutate { remove_field => ["@version","host","message","path"]  }

   if [type] == "inbound" {
       aggregate { 
                      task_id => "%{FIXinID}"
                      code => "map['time1_a']=event.get('TIME1');"
                      map_action => "create"
                          }
    }
  
    if [type] == "outbound" {
           aggregate { 
                      task_id => "%{FIXinID}"
                      code => "event.set( 'time6_a', event.get('TIME6') );   event.set( 'time1_a', map['time1_a'] );"
                      map_action => "update"
                      end_of_task =>true
                      timeout => 120
                          }
    }
}

The combination result sometimes is perfect that all events merge successfully except the alone one FIXinID=55935. However, sometimes only 3 or 2 inbound/outbound events can combine together.

Have you set pipeline.workers to 1? Are you running a version where you need to disable java_execution (anything from 7.0 to 7.6).

I tried "-w 1" in my execution cmd, it worked! Thanks Badger so much!