My apologies for the late reply. We are using Logstash 7.16.3 image and the Helm chart from Bitnami.
Regarding the plain codec for Beats input, if I have already have the codec => json configured, will adding the codec => plain be causing any issue? Unfortunately, I am not familiar with configuring the pipelines.yml/logstash.yml files in the Helm files nor with building them into the image.