packetbeat is not catching large http POST packets
If the HTTP stream becomes larger than ~10MB then Packetbeat drops it.
The max size is defined in a the TCP_MAX_DATA_IN_STREAM constant which cannot be changed at runtime.
If you enable debug for Packetbeat you should be able to confirm that the stream is being dropped by looking for "Stream data too large, dropping TCP stream" in your logs. Run Packetbeat with -d "http" to enable this debug output.