Well the event that goes to rabbitmq will not have computer_tag since that mutate is only applied to the events that are discarded, not the event created by aggregate. Move the mutate after the aggregate.
Generally your aggregate looks good. I would add event.cancel to the code option of the aggregate and change push_previous_map_as_event to push_map_as_event_on_timeout
Which logstash version. I wonder if you are hitting this issue.