The knowledge provided in this KB won't work: How to Automatically Set Issue Security Level for Sub Task During Creation.

It is expected that you can't set a issue security level to a sub-task. Sub-tasks will only inherit security levels from their parent tasks.

There are suggestions for allowing that: JRASERVER-5869, JRACLOUD-5869.
Notice that they have been set as won't fix.

So an add-on should not be able to bypass that condition from Jira.