I may be hitting the same issue here.  I'm on revision 96 (deliberately for the sake of alignment with another cloud), but I've pulled the deployed sources for rev 96 and compared to a downloaded rev 97 nrpe charm and the sources appear the same.

"lsblk -J" output is as follows:

# lsblk -J
{
   "blockdevices": [
      {"name": "loop0", "maj:min": "7:0", "rm": "0", "size": "55.6M", "ro": "1", "type": "loop", "mountpoint": "/snap/core18/2714"},
      {"name": "loop1", "maj:min": "7:1", "rm": "0", "size": "6M", "ro": "1", "type": "loop", "mountpoint": "/snap/prometheus-grok-exporter/1"},
      {"name": "loop2", "maj:min": "7:2", "rm": "0", "size": "116.8M", "ro": "1", "type": "loop", "mountpoint": "/snap/core/14784"},
      {"name": "loop3", "maj:min": "7:3", "rm": "0", "size": "21.8M", "ro": "1", "type": "loop", "mountpoint": "/snap/prometheus-libvirt-exporter/36"},
      {"name": "loop4", "maj:min": "7:4", "rm": "0", "size": "9M", "ro": "1", "type": "loop", "mountpoint": "/snap/canonical-livepatch/164"},
      {"name": "loop5", "maj:min": "7:5", "rm": "0", "size": "116.8M", "ro": "1", "type": "loop", "mountpoint": "/snap/core/14946"},
      {"name": "loop6", "maj:min": "7:6", "rm": "0", "size": "55.6M", "ro": "1", "type": "loop", "mountpoint": "/snap/core18/2721"},
      {"name": "sda", "maj:min": "8:0", "rm": "0", "size": "894.2G", "ro": "0", "type": "disk", "mountpoint": null,
         "children": [
            {"name": "crypt-<redacted>", "maj:min": "253:0", "rm": "0", "size": "894.2G", "ro": "0", "type": "crypt", "mountpoint": "/var/lib/nova/instances"}
         ]
      },
      {"name": "sdb", "maj:min": "8:16", "rm": "0", "size": "447.1G", "ro": "0", "type": "disk", "mountpoint": null,
         "children": [
            {"name": "sdb1", "maj:min": "8:17", "rm": "0", "size": "572M", "ro": "0", "type": "part", "mountpoint": "/boot/efi"},
            {"name": "sdb2", "maj:min": "8:18", "rm": "0", "size": "18.6G", "ro": "0", "type": "part", "mountpoint": "/"},
            {"name": "sdb3", "maj:min": "8:19", "rm": "0", "size": "4.7G", "ro": "0", "type": "part", "mountpoint": "/home"},
            {"name": "sdb4", "maj:min": "8:20", "rm": "0", "size": "4.7G", "ro": "0", "type": "part", "mountpoint": "/tmp"},
            {"name": "sdb5", "maj:min": "8:21", "rm": "0", "size": "223.5G", "ro": "0", "type": "part", "mountpoint": "/var"},
            {"name": "sdb6", "maj:min": "8:22", "rm": "0", "size": "93.1G", "ro": "0", "type": "part", "mountpoint": "/var/log"},
            {"name": "sdb7", "maj:min": "8:23", "rm": "0", "size": "4.7G", "ro": "0", "type": "part", "mountpoint": "/var/tmp"},
            {"name": "sdb8", "maj:min": "8:24", "rm": "0", "size": "93.1G", "ro": "0", "type": "part", "mountpoint": "/var/log/audit"}
         ]
      }
   ]
}


Upon upgrading to this version of the nrpe charm, I have 4 check_space alerts in nagios which return an UNKNOWN status with status message "NRPE: Command 'check_space_$(path)' not defined.

space_check is set to the default, which is:

    default: |-
      check: auto
      auto_params: -w 25% -c 20% -K 5%


With that, I get these checks coming through OK:

check_space_home (/home)
check_space_root (/)
check_space_tmp (/tmp)
check_space_var (/var)

While these checks appear to not be defined properly:

check_space_var_lib_nova_instances
check_space_var_log
check_space_var_log_audit
check_space_var_tmp