[Jack-Devel] Zombified process without any CPU load

I am getting this error "Jack Error: zombified - calling shutdown handler" with default timeout of 500ms. Increasing timeout to 4000ms helps, but it isn't clear why 500ms isn't sufficient. Jack thread only connects with the actual client code through the process() callback. I timed process(), it never takes longer than 5ms. Given that the system has very low CPU load, why does the process become zombified? And why other apps, like Ardour, don't suffer from this? Client: newly written Jack backend for qTox. jack-0.125.0 on FreeBSD 11. Yuri