So this is an artifact of maven dependency loading. Apparently logstash-logback-encoder-4.6.jar is pinned to logback-core-1.1.3 and that version of core does not contain this class. It appears as though you have moved the class from logback-classic to logback-core sometime between version 1.1.3 and version 1.1.6. But for whatever reason classic doesn't force a binding to core properly.