I'm using v6.4.0 (for the full stack). I looked at .watcher_history and it looks like nothing had been triggered since my maintenance work on the cluster at the start of December. There were a few attempted executions like this:
watch_id:xJSF3zE-RBCV5j2uI5XEPA_kibana_version_mismatch node:Jwyj0hcGQeSlTvIUUXViAQ state:not_executed_already_queued trigger_event.type:schedule trigger_event.triggered_time:December 28th 2018, 08:42:51.319 trigger_event.schedule.scheduled_time:December 28th 2018, 08:42:51.124 messages:Watch is already queued in thread pool _id:xJSF3zE-RBCV5j2uI5XEPA_kibana_version_mismatch_04c8d92e-e7c8-4431-b694-1c8a1db37802-2018-12-28T13:42:51.319Z _type:doc _index:.watcher-history-9-2018.12.28 _score: -
But none of the other system watches seem to have executed. Since doing a stop and start on watcher, it looks like the system watches are executing so I'm waiting to see if the problem is resolved.