"run script" hook leaves behind defunct/zombie processes

The "run script" hook (libdhcp_run_script.so) leaves behind defunct/zombie processes after calling the desired script.

ps aux | grep defunct
_kea     2816177  0.0  0.0      0     0 ?        Z    12:27   0:00 [kea-dhcp-nsupda] <defunct>
_kea     2816269  0.0  0.0      0     0 ?        Z    12:28   0:00 [kea-dhcp-nsupda] <defunct>
_kea     2816382  0.0  0.0      0     0 ?        Z    12:30   0:00 [kea-dhcp-nsupda] <defunct>
_kea     2816413  0.0  0.0      0     0 ?        Z    12:30   0:00 [kea-dhcp-nsupda] <defunct>

I tried on different machines and environments (Debian, RedHat, KEA 1.9.5, 1.9.6...).

My /etc/kea/kea-dhcp4.conf config (only relevant part):

"hooks-libraries": [
            {
                "library": "/usr/lib/x86_64-linux-gnu/kea/hooks/libdhcp_stat_cmds.so"
            },
            {
                "library": "/usr/lib/x86_64-linux-gnu/kea/hooks/libdhcp_lease_cmds.so"
            },
            {
                "library": "/usr/lib/x86_64-linux-gnu/kea/hooks/libdhcp_run_script.so",
                "parameters": {
                    "name": "/kea-dhcp-nsupdate-hook.sh",
                    "sync": false
                }
            }
        ],

version:

kea-dhcp4 -v                  
1.9.6

Could the reason be the way ProcessSpawn is called by "run_script" ? run_script.cc#L54

void
RunScriptImpl::runScript(const ProcessArgs& args, const ProcessEnvVars& vars) {
    ProcessSpawn process(io_service_, name_, args, vars);
    process.spawn(true);
}

process_spawn.cc#L270

pid_t
ProcessSpawnImpl::spawn(bool dismiss) {
    lock_guard<std::mutex> lk(mutex_);
    // Create the child
    pid_t pid = fork();
    if (pid < 0) {
        isc_throw(ProcessSpawnError, "unable to fork current process");

    } else if (pid == 0) {
        // Run the executable.
        execve(executable_.c_str(), args_.get(), vars_.get());
        // We may end up here if the execve failed, e.g. as a result
        // of issue with permissions or invalid executable name.
        _exit(EXIT_FAILURE);
    }

    // We're in the parent process.
    if (!dismiss) {
        store_ = true;
        process_collection_[this].insert(std::pair<pid_t, ProcessStatePtr>(pid, ProcessStatePtr(new ProcessState())));
    }
    return (pid);
}

Since dismiss is always set to true the process state won't be stored. If the child calls the SIGCHLD handler the parent process will not recognize. Am I totally wrong? Could someone shine light on this issue please. Thank you!