It seems like I might be affected by this recent qemu 4.1 bug:
https://bugs.archlinux.org/task/64150
https://bugzilla.redhat.com/show_bug.cgi?id=1764721
https://lists.gnu.org/archive/html/qemu-block/2019-10/msg01414.html
I'm not 100% sure. But the second link mentions the easiest way to reproduce this bug
is by using snapshots, which lines up with my experience.