So, disoriented pilot, and nothing wrong with the aircraft itself, is the conclusion. (Video link should work for a while.)
https://www3.nhk.or.jp/nhkworld/en/news/20190610_21/
Japan's Air Self-Defense Force says a stealth fighter pilot who crashed off northern Japan failed to detect the jet's rapid descent because it is highly likely he was disoriented. The state-of-the-art F-35A went down off Aomori Prefecture during a training mission in April. The pilot died.
The ASDF says in its report that it analyzed the plane's flight route using data from other F-35As that took part in the training mission. The report says analysis shows the fighter jet descended at a speed of about 1,000 kilometers per hour from an altitude of more than 9,000 meters for roughly 30 seconds just before the accident. The report also says the pilot sent a message that appeared to tell the other aircraft to proceed to the next part of the training, but he did not signal any problems. It says there are no signs that he attempted an emergency evacuation.
The ASDF says it is highly unlikely the accident was caused by mechanical failure and it is considering resuming flight operations of the remaining 12 F-35As.