QThreadPool produces zombie [Thread (pooled)] threads

Since about 2019-10-10, when we launched production applications rebuilt with Qt 5.13.1 (now 5.13.2) after upgrading from 5.9.x, we started seeing zombie threads slowly popping up (brownish color):

Currently, it looks like this via ps and pstree (first two threads in pstree are zombies):

$ ps aux | grep -v grep | grep Zs
root     14582  0.0  0.0      0     0 ?        Zs   Jan15   0:00 [Thread (pooled)] <defunct>
root     14591  0.0  0.0      0     0 ?        Zs   Jan15   0:00 [Thread (pooled)] <defunct>

 
GPTScheduler(31254)─┬─Thread (pooled)(14582) 
                    ├─Thread (pooled)(14591) 
                    ├─{Thread (pooled)}(14578) 
                    ├─{Thread (pooled)}(14580) 
                    └─{Thread (pooled)}(14586)

I do not know how to reproduce it with smaller application. What QRunnables are doing, is either invoking QProcess p; ..., p.startDetached();,(launches PHP process), or executing some QSqlQuery. Zombies are not seem to be produced on every QThreadPool::globalInstance()->start(foo); - as seen in munin graph, the growth of zombie "garden" is rather slow. Looking at the logs and munin graph, it seems as if zombies, at least these latest zombies seen in ps output provided, where produce on idle moment, after almost an hour after doing any work.