Conditional file output with overwrite leads to empty file

[nicklan (Nick)]

I have a file output that is triggered conditionally set up as:

output {
  if [@metadata][dofile] {
    file {
      path => "/tmp/ts.out"
      codec => line { format => "%{[afield]}"}
      write_behavior => "overwrite"
    }
  }
  ...
}

This works sometimes, but what I observe is that most of the time the file is empty. It seems that if the conditional doesn't match the file is just truncated. If I tail -f the file I see all the expected data, but doing a cat at random points almost never shows things.

help!