The issue is that the field already exists. The code does different things depending on the type of the existing field. If the existing field is a LogStash::Timestamp then it logs a DEBUG message saying it will leave it unmodified.
[2020-09-11T13:50:33,173][DEBUG][logstash.filters.grok ][main][91fb85f2e5abc66c776cd383ec1ba5840bbc036e454a5930ae9c1db8f095cefe] Not adding matched value - found existing (LogStash::Timestamp) {:field=>"@timestamp", :value=>"2020-08-20 20:10:27"}
Now you might think that you could get around this by adding
overwrite => [ "@timestamp" ]
to your grok filter, but that does not work. logstash knows that @timestamp should be a LogStash::Timestamp and when grok tries to event.set it to a string it gets a WARN:
Grok regexp threw exception {:exception=>"wrong argument type String (expected LogStash::Timestamp)"
Change your grok filter to start with '%{TIMESTAMP_ISO8601:[@metadata][timestamp]} and parse that using a date filter.
date { match => [ "[@metadata][timestamp]", "YYYY-MM-dd HH:mm:ss" ] }