It's been claimed a video by a US submariner that the Soviet submarine K-10 collided with an unknown submarine in 1983 and this likely was a Chinese one ... because two years later the Chinese press supposedly published an obituary admitting to the loss of the sub, or at least the loss of a crew.
Furthermore, there's a web page that claims that in 1985 China lost nothing short of a nuclear-powered ballistic missile submarine (SSBN) of the Xia class.
It was reported that a second unit was constructed but lost with all hands in an accident in 1985, but Chinese secrecy remains at Cold War levels.
Wikipedia doesn't list any Chinese sub accidents until after the year 2000 (and also says that only one Xia was ever built.)
So, is there any other confirmation/evidence that China lost a sub (likely with all hands on board) during 1983-1985?