Yep, you had an event where the @datestamp didn't grok. The fix depends on your data. I encountered it in Apache logs that appear to be from ddos attempts. I just added code something like if no @datestamp, make it "now".
I had extracted the filebeat ingest pipeline for use in logstash, but the same problem probably occurs if using ES ingest mode.
Also, if you have trouble seeing these events in Kibana, create a index pattern w/o using a timestamp. A name like "filebeat-7*" to be different from the "filebeat*" you probably have now.
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