Using filters on loggin operator flows

I´m trying to set up central logging with the new logging system in Rancher 2.5.7. I have defined a syslog output, that sends to my papertrail logging system. My problem is, that each line of log output fram a container is packed in a JSON-Object along wth lot´s of metadata, but I only want to see the original log entry from the container in my papertrail log for readability and log data size. Each event looks like

“log”: …
“stream”: …
“time”: …


but I only need the “log” key. So I tried to modify the data using filters on the flow for the container logs like:

   - record_transformer:
      keep_keys: log 

but nothing changes in log output. I also tried other filters, but that doesn´t seem to take any effect on the log output.

in the meantime I found out, that using the “record_modifier”-filter instead of “record_transformer” is workling so it seems to be an issue with record_transformer.