KB4534309 is the security-only update.
The corresponding monthly rollup is KB4534297, which contains all the updates from KB4534309 as well as additional quality updates.
Generally, Microsoft does not build additional blockers in the security-only updates to prevent them from being installed if the corresponding monthly rollup is already on the system.
So first thing to check is to see if KB4534297 is installed.
If the system already has monthly rollup KB4534297 installed, then the fixlet for KB4534309 will not be relevant by design since it is not needed.