It actually makes tons of sense that while the thread is running, that the object representing it is alive. After the thread finishes its work, the object dies.

>>> import time, threading, weakref, gc
>>> t = threading.Thread(target=time.sleep, args=(10,))
>>> wr = weakref.ref(t)
>>> t.start()
>>> del t
>>> gc.collect()
>>> wr()
<Thread(Thread-1, started 139937234327296)>
Wait 10 seconds...
>>> gc.collect()
>>> wr()

The thread is gone (which doesn't happen with the pool).

Anyhow, I've submitted a patch to fix the bug that was introduced 9 years ago on GH, feel free to check it.