This seemed to be a problem in the earlier on device sleep beta versions but had not seen it in more than a week. Today sleep detection worked and showed, admittedly strange, sleep feedback (seems to say that 1.5 hours of REM is adequate while 3 hours the previous day is inadequate...??).
Went for a run and when I got back the sleep data had disappeared and in its place on the widget I have “No Sleep Data Recorded”.
Anyone else also still getting this occasionally after the device showing sleep in the morning?