web-application-gateway-firewall logs gives jsonparseerror

[kp2822]

web-application-gateway-Access and web-application-gateway-Performance logs works without issues.
But, web-application-gateway-firewall logs gives jsonparseerror

i use the following sanitized input and filter plugin configuration.

azureblob
{
storage_account_name => "account-name"
storage_access_key => "**********************"
container => "container-name"
file_head_bytes => 0
file_tail_bytes => 0
}
}

filter {
json {
source => "message"
}
date {
match => ["time", "ISO8601"]
}
}