Steps to reproduce:

1. Play audiobook
2. Set sleep timer to 15 min
3. Continue to listen to verify still playing and countdown is working
4. Turn device power off while sleep-timer is still on 
5. Turn Power back on

Audio will continue to play in sleep-timer mode, as expected.

But the lockscreen is blank: it doesn't show times, book info, 15-sec-skip buttons, or status/scrubber.