APM server logs contain document body from a partially failed bulk index request. For example, in case of unavailable_shards_exception for a specific document, since the ES response line contains the document body, and that APM server logs the ES response line on error, the document is effectively logged.
Secret token configuration is never applied when using ECK <2.8 with APM Server >=8.0. This could lead to anonymous requests to an APM Server being accepted and the data ingested into this APM deployment.