Others have answered why this isn't a memory leak as such and is not as big a deal as you may think.
But if you are still concerned, you can reduce it, even if doing so is a bad idea.
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You're running it natively which means you're probably using a systemd .service file to manage jackett. Research the .system setting "RuntimeMaxSec" - that will force a restart of the service every N seconds and prevent it growing. (This is a bad idea, but if you want to boss it around, you can)
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Run it in docker and force a max memory setting. Docker will prevent it using more than you set. You can also restrict cpu usage this way too. docker-compose example goes something like:
deploy: resources: limits: cpus: 0.5 memory: 100m