This problem is being discussed in kernel mailing list right now: https://www.spinics.net/lists/linux-block/msg29103.html (its cause is more or less known now)
- Bug#900442: linux-image-4.16.0-2-amd64: LVM2 COW snap... WGH
- Bug#900442: linux-image-4.16.0-2-amd64: LVM2 COW... Salvatore Bonaccorso