PluginProcessor, as of 2.12.1, writes status messages to System.out. This breaks using the Log4J annotation processor with Bazel, as it expects those messages to be sent to javax.annotation.processing.Messager. PluginProcessor on master uses Messager. This ticket is to track backporting just that change to either 2.12.2 or 2.13.0, whatever makes the most sense.
See this discussion on the mailing list for more context.