Combining 1) and 2) from #7 the results are ok for both bugs (I'm preparing already the MP):


root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10 -c 10
DISK OK - free space: / 6971 MB (85% inode=99%); /dev/fuse 32071 MB (100% inode=99%); /dev/net/tun 32071 MB (100% inode=99%); /dev/full 32071 MB (100% inode=99%); /dev/null 32071 MB (100% inode=99%); /dev/random 32071 MB (100% inode=99%); /dev/tty 32071 MB (100% inode=99%); /dev/urandom 32071 MB (100% inode=99%); /dev/zero 32071 MB (100% inode=99%); /snap 6971 MB (85% inode=99%);| /=1204MB;8165;8165;0;8175 /dev/fuse=0MB;32061;32061;0;32071 /dev/net/tun=0MB;32061;32061;0;32071 /dev/full=0MB;32061;32061;0;32071 /dev/null=0MB;32061;32061;0;32071 /dev/random=0MB;32061;32061;0;32071 /dev/tty=0MB;32061;32061;0;32071 /dev/urandom=0MB;32061;32061;0;32071 /dev/zero=0MB;32061;32061;0;32071 /snap=1204MB;8165;8165;0;8175

root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -p /reproduce-tmpfs
DISK OK - free space: /reproduce-tmpfs 2048 MB (100% inode=99%);| /reproduce-tmpfs=0MB;1843;1945;0;2048

root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -N tmpfs
DISK OK - free space: /dev 0 MB (99% inode=99%); /dev/lxd 0 MB (100% inode=99%); /dev/.lxd-mounts 0 MB (100% inode=99%); /proc/sys/kernel/random/boot_id 0 MB (99% inode=99%); /dev/shm 32106 MB (100% inode=99%); /run 6421 MB (99% inode=99%); /run/lock 5 MB (100% inode=99%); /sys/fs/cgroup 32106 MB (100% inode=99%); /run/user/0 6421 MB (100% inode=99%); /run/snapd/ns 6421 MB (99% inode=99%); /reproduce-tmpfs 2048 MB (100% inode=99%); /reproduce-squashfs 2048 MB (100% inode=99%); /reproduce-tracefs 2048 MB (100% inode=99%);| /dev=0MB;0;0;0;0 /dev/lxd=0MB;0;0;0;0 /dev/.lxd-mounts=0MB;0;0;0;0 /proc/sys/kernel/random/boot_id=0MB;0;0;0;0 /dev/shm=0MB;28895;30500;0;32106 /run=0MB;5778;6099;0;6421 /run/lock=0MB;4;4;0;5 /sys/fs/cgroup=0MB;28895;30500;0;32106 /run/user/0=0MB;5778;6099;0;6421 /run/snapd/ns=0MB;5778;6099;0;6421 /reproduce-tmpfs=0MB;1843;1945;0;2048 /reproduce-squashfs=0MB;1843;1945;0;2048 /reproduce-tracefs=0MB;1843;1945;0;2048

root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -N tmpfs -p /reproduce-tmpfs
DISK OK - free space: /reproduce-tmpfs 2048 MB (100% inode=99%);| /reproduce-tmpfs=0MB;1843;1945;0;2048

root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -X tmpfs -p /reproduce-tmpfs
DISK UNKNOWN - free space:|

root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -X squashfs -p /reproduce-tmpfs
DISK OK - free space: /reproduce-tmpfs 2048 MB (100% inode=99%);| /reproduce-tmpfs=0MB;1843;1945;0;2048
root@nagios-focal-miriam:~# /usr/lib/nagios/plugins/check_disk -w 10% -c 5% -X squashfs -N tmpfs

DISK OK - free space: /dev 0 MB (99% inode=99%); /dev/lxd 0 MB (100% inode=99%); /dev/.lxd-mounts 0 MB (100% inode=99%); /proc/sys/kernel/random/boot_id 0 MB (99% inode=99%); /dev/shm 32106 MB (100% inode=99%); /run 6421 MB (99% inode=99%); /run/lock 5 MB (100% inode=99%); /sys/fs/cgroup 32106 MB (100% inode=99%); /run/user/0 6421 MB (100% inode=99%); /run/snapd/ns 6421 MB (99% inode=99%); /reproduce-tmpfs 2048 MB (100% inode=99%); /reproduce-squashfs 2048 MB (100% inode=99%); /reproduce-tracefs 2048 MB (100% inode=99%);| /dev=0MB;0;0;0;0 /dev/lxd=0MB;0;0;0;0 /dev/.lxd-mounts=0MB;0;0;0;0 /proc/sys/kernel/random/boot_id=0MB;0;0;0;0 /dev/shm=0MB;28895;30500;0;32106 /run=0MB;5778;6099;0;6421 /run/lock=0MB;4;4;0;5 /sys/fs/cgroup=0MB;28895;30500;0;32106 /run/user/0=0MB;5778;6099;0;6421 /run/snapd/ns=0MB;5778;6099;0;6421 /reproduce-tmpfs=0MB;1843;1945;0;2048 /reproduce-squashfs=0MB;1843;1945;0;2048 /reproduce-tracefs=0MB;1843;1945;0;2048

root@nagios-focal-miriam:~# df -h
Filesystem                              Size  Used Avail Use% Mounted on
default/containers/nagios-focal-miriam  8.0G  1.2G  6.9G  15% /
none                                    492K  4.0K  488K   1% /dev
udev                                     32G     0   32G   0% /dev/tty
tmpfs                                   100K     0  100K   0% /dev/lxd
tmpfs                                   100K     0  100K   0% /dev/.lxd-mounts
tmpfs                                    32G     0   32G   0% /dev/shm
tmpfs                                   6.3G  224K  6.3G   1% /run
tmpfs                                   5.0M     0  5.0M   0% /run/lock
tmpfs                                    32G     0   32G   0% /sys/fs/cgroup
tmpfs                                   6.3G     0  6.3G   0% /run/user/0
tmpfs                                   2.0G     0  2.0G   0% /reproduce-tmpfs
squashfs                                2.0G     0  2.0G   0% /reproduce-squashfs
tracefs                                 2.0G     0  2.0G   0% /reproduce-tracefs