Abbreviation CST timezone issue when use postgresql filebeat module

[yanhj93 (haojie yan)]

    filebeat.modules:
    - module: postgresql
      log:
        enabled: true
        var.paths: ["/data/pgdata/pg_log/*.log"]
        input:
          tags: "server"
          processors:
          - drop_fields:
              fields: ["event.timezone"]
              ignore_missing: true
          - add_locale:
              format: offset

I was using the postgresql module to parse postgresql logs(filebeat version was 7.14). The problem was @timestamp of log document in es after processed by the pipeline was parsed by incorrect event.timezone(which is CST which is ambiguous). actually the server timezone was +0800.
I have tried a log of ways include above(I noticed the postgresql pipeline parse timestamp with timezone specified in event.timezone) and didn't change the result the event.timezone was CST, and @timestamp was ahead five hours. So I came here to get some help.
What I expected was let the pipeline parse timestamp with right timezone(+0800).

[yanhj93 (haojie yan)]

And I have no idea where does the CST come. The server timezone and database timezone are all set to Asia/Shanghai.