Customer uses custom monitor with perl script.
We find the Last Response shows as "Failure-UP" but the Status is "UP".

For the "Last Response", NetScaler retrieves the output generated by the script during execution.
If the output includes any string, NetScaler will display that string in the GUI.

From log, we can see the output of the script is "UP".

vi var/nslognsumond.log
Wed Jun  4 11:09:20 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:21 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:21 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:21 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:21 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:22 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)
Wed Jun  4 11:09:22 2025: /nsconfig/monitors/tcp_monitor.pl Exit Reason : (UP) (Partition ID: 0)

Change the following string in custom perl script 

from

return (0, "UP");  

To

return (0); 

ADC13.1- Last Response shows as "Failure-UP" even the Status is "UP" in Netscaler Custom Monitor