On Fedora Rawhide x64-64, I can reproduce the test_ftplib test_makeport() issue in reliable way.

On Python 3.9, the test also logs the exception but the test is marked as a success:
---
$ ./python -m test -u all test_ftplib -v -m test_makeport
(...)
test_makeport (test.test_ftplib.TestFTPClass) ... ok
test_makeport (test.test_ftplib.TestIPv6Environment) ... ok
test_makeport (test.test_ftplib.TestTLS_FTPClassMixin) ... Exception in thread Thread-3:
Traceback (most recent call last):
  File "/home/vstinner/python/3.9/Lib/asyncore.py", line 83, in read
    obj.handle_read_event()
  File "/home/vstinner/python/3.9/Lib/test/test_ftplib.py", line 377, in handle_read_event
    self._do_ssl_handshake()
  File "/home/vstinner/python/3.9/Lib/test/test_ftplib.py", line 338, in _do_ssl_handshake
    self.socket.do_handshake()
  File "/home/vstinner/python/3.9/Lib/ssl.py", line 1309, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLZeroReturnError: TLS/SSL connection has been closed (EOF) (_ssl.c:1129)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vstinner/python/3.9/Lib/threading.py", line 973, in _bootstrap_inner
    self.run()
  File "/home/vstinner/python/3.9/Lib/test/test_ftplib.py", line 291, in run
    asyncore.loop(timeout=0.1, count=1)
  File "/home/vstinner/python/3.9/Lib/asyncore.py", line 207, in loop
    poll_fun(timeout, map)
  File "/home/vstinner/python/3.9/Lib/asyncore.py", line 150, in poll
    read(obj)
  File "/home/vstinner/python/3.9/Lib/asyncore.py", line 87, in read
    obj.handle_error()
  File "/home/vstinner/python/3.9/Lib/test/test_ftplib.py", line 414, in handle_error
    raise Exception
Exception
ok
(...)
Tests result: SUCCESS
---

make buildbottest uses -W option which hides the test output unless the test failed.

In this case, the unhandled threading exception is siently ignored:
---
$ ./python -m test -u all test_ftplib -W -m test_makeport -j1
(...)
0:00:00 load avg: 0.75 [1/1] test_ftplib passed
(...)
Tests result: SUCCESS
---

On Python 3.10 and newer, libregrtest sets a threading excepthook to (1) log the exception (2) mark that the note "altered the environmen": mark the test as "ENV CHANGED". I did that to detect unhandled exceptions like this one.

In this case, unhandled exceptions come from the four handle_error() method of test_ftplib dispatcher classes, like SSLConnection, which are implemented as:

    def handle_error(self):
        raise Exception

test_ftplib closes sockets in TestFTPClass.tearDown() method:

    def tearDown(self):
        self.client.close()
        self.server.stop()
        # Explicitly clear the attribute to prevent dangling thread
        self.server = None
        asyncore.close_all(ignore_all=True)

The problem is that the code doesn't implement any kind of error handling. If asyncore gets a socket exception, it calls handle_error() which raises an a new Exception instance. That's it.

IMO it's not worth it to bother with handling socket errors in test_ftplib. The server is implemented with asyncore which is deprecated since Python 3.6. The FTP protocol itself is legacy.

Attached PR simply ignores socket erors rather than logging unhandled threading exceptions.