Possibly related to issues described here?  I can’t find anything relating to cause yet. https://www.news.com.au/technology/innovation/military/f35-troubles-pentagon-refuses-to-accept-new-aircraft-as-its-makes-problem-report-top-secret/news-story/220610ce01157b1c32dbc947135a3b26 https://www.defensenews.com/air/2018/06/05/watchdog-group-to-pentagon-fix-f-35-tech-problems-before-full-rate-production-starts/