Addendum: This never happened during half a year of using Debian 9. It did happen on GuixSD half a year ago.
This happens on linux-libre-4.9 (same Linux base version as Debian 9) as well as on what recently was current linux-libre (which I can no longer use with my GPU since a few weeks ago). I therefore believe the kernel to be unrelated, unless there are some patches in Debian but not in recent Linux. Regards, Florian
