One thing you could try is to run the status command manually from the CLI.
This appears to be the one to run. Replace the "$node_ip:9999" section with the proper IP and port, and without the extra options from the script, it may give more detail on whether the check actually functions.
sh /config/monitors/denodo-db-tools/bin/ping.sh -t 2000 -v $node_ip:9999
You may also be able to use some of the example external monitor here: https://devcentral.f5.com/wiki/AdvDesignConfig.TemplateForExternalLtmMonitors.ashx
Another thing to consider is whether you need to strip the IPv6 prefix from the node IP. You can find an example of that in the script from the link above.