Comment 2 for bug 1940916

Thanks for reporting this issue.

I could reproduce the issue in fresh lxc containers in focal and in impish, although it is indeed not present in bionic:

apt update
apt install -y monitoring-plugins
mkdir /reproduce
mount -t tmpfs -o rw,size=2G tmpfs /reproduce
/usr/lib/nagios/plugins/check_disk -w 10% -c 5% -p /reproduce
DISK UNKNOWN - free space:|