Running snap LXD on Ubuntu 18.04 (4.15.0-76-generic), it looks like that after an upgrade to 3.22 containers broke similar to this issue https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=920985
Simply restarting the containers seems to fix the issue.
Am not sure how to get lxcfs.service logs from snap LXD so I can’t post any relevant logs.
EDIT: I was wrong, simply restarting the container only gets things half-working.
# df -h df: /var/lib/lxcfs: Transport endpoint is not connected <rest of normal df output>