How to disable JSON parser

I have different logs from applications, and I want sent it to elasticsearch.
In some cases in logs appear JSON. I want to send it in elasticsearch as plaintext. But in log logstash I see "JSON parse failure":

[2021-07-22T07:08:51,808][ERROR][logstash.inputs.gelf     ] JSON parse failure. Falling back to plain-text {:error=>#<LogStash::Json::ParserError: Unexpected character ('f' (code 102)): was expecting comma to separate Object entries
 at [Source: (byte[])"{"facility":"fluentd","protocol ...

In input section I use codec => "plain" .
In filter section I not use any JSON modificators.

How I can wholly disable JSON parser for this pipeline?

my logstash config:

input {
  gelf {
    use_tcp => true
    port => 5046
    remap => false
    tags => ["cicd-k8s-gelf"]
	codec => "plain"

filter {
  ruby { code => "event.set('@orig_timestamp' ," }

  if "cicd-k8s-gelf" in [tags] {
    ruby {
      code => '
        t ="@orig_timestamp").to_i)
        t2 ="@timestamp").to_i)
        event.set("[@metadata][d_t]", t.strftime("%Y.%m.%d")) 
        event.set("@diff_time", t-t2)

output {
  if "cicd-k8s-gelf" in [tags] {
    elasticsearch {
      hosts                        => ["ELK_SERVERS"]
      index                        => "INDEX-%{[@metadata][d_t]}"
      user                         => "${USER}"
      password                     => "${PWD}"
      manage_template              => false
      ilm_enabled                  => false

My reading of the code is that it unconditionally calls the JSON parser, so no, you cannot disable it.

I understand, thanks