terminal – How to use launchd to run a process longer than the interval?

I’ve written a launch agent:

    
        Label
        local.mylaunchagent.plist
        ProgramArguments
        
            ~/my_script.sh
        
        StartInterval
        12
        StandardOutPath
        ~/my_log_file
        StandardErrorPath
        ~/my_err_file
        RunAtLoad
        
    

It works, but it runs every 22 seconds instead of every 12 seconds, because ~/my_script.sh takes 10 seconds to finish.

launchd seems to be adding the interval time and the execution time together, even though I want the program to be run every interval. I don’t want launchd to wait for it to finish.

So I tried running the program in the background:

ProgramArguments

    /bin/zsh
    -c
    ~/my_script.sh & disown

But now I don’t get any output at all, so I suspect that launchd kills all spawned processes as soon as the process that it launched exits.

So my question is, how can I start a slow command (takes an unpredictable 10+ seconds to finish) every 12 seconds reliably?