DK2 is freezing with last frame on quit

This may be 5.1.2f related but when I exit my game the screen on the DK2 freezes and stays with the last frame visible permanently, even though the direct to rift window on my main monitor has gone (and there is no evidence of the app still running). It should go black upon quitting and the light should go yellow, instead it says blue. This happens with both Alt-F4 and Application.Quit().

Anybody else experiencing this? Could I be doing something wrong to achieve this?