Of the test  test_plugin_with_error seems the most simple and even if the same is executed twice the second one fails.
So i leaves something yet unknown behind in the test environment that breaks the second execution
.



brz selftest: /build/breezy-xbGxjH/breezy-3.2.1/brz
   /build/breezy-xbGxjH/breezy-3.2.1/breezy
   bzr-3.2.1 python-3.9.6 Linux-5.4.0-73-generic-x86_64-with-glibc2.33

test_plugins.TestLoadingPlugins.test_plugin_with_error                                                                                                                          OK       25ms
test_plugins.TestLoadingPlugins.test_plugin_with_error2                                                                                                                       FAIL        5ms
    log: {{{
1.650  loading plugins!
1.650  using _Path('breezy.testingplugins', [], [], ['.'])
1.650  Traceback (most recent call last):
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/plugin.py", line 430, in _load_plugin_module
    __import__(_MODULE_PREFIX + name)
ModuleNotFoundError: No module named 'breezy.testingplugins.some_error'

1.650  Unable to load plugin 'some_error' from '.': No module named 'breezy.testingplugins.some_error'
 WARNING  Unable to load plugin 'some_error' from '.': No module named 'breezy.testingplugins.some_error'
1.652  removed breezy.testingplugins from sys.modules
}}}

Traceback (most recent call last):
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/test_plugins.py", line 340, in test_plugin_with_error2
    self.assertEqual(
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/__init__.py", line 1307, in assertEqual
    raise AssertionError("%snot equal:\na = %s\nb = %s\n"
AssertionError: not equal:
a = "Unable to load plugin 'some_error' from '.': bad\n"
b = ("Unable to load plugin 'some_error' from '.': No module named "
 "'breezy.testingplugins.some_error'\n")

======================================================================
FAIL: breezy.tests.test_plugins.TestLoadingPlugins.test_plugin_with_error2
----------------------------------------------------------------------
Traceback (most recent call last):
testtools.testresult.real._StringException: log: {{{
1.650  loading plugins!
1.650  using _Path('breezy.testingplugins', [], [], ['.'])
1.650  Traceback (most recent call last):
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/plugin.py", line 430, in _load_plugin_module
    __import__(_MODULE_PREFIX + name)
ModuleNotFoundError: No module named 'breezy.testingplugins.some_error'

1.650  Unable to load plugin 'some_error' from '.': No module named 'breezy.testingplugins.some_error'
 WARNING  Unable to load plugin 'some_error' from '.': No module named 'breezy.testingplugins.some_error'
1.652  removed breezy.testingplugins from sys.modules
}}}

Traceback (most recent call last):
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/test_plugins.py", line 340, in test_plugin_with_error2
    self.assertEqual(
  File "/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/__init__.py", line 1307, in assertEqual
    raise AssertionError("%snot equal:\na = %s\nb = %s\n"
AssertionError: not equal:
a = "Unable to load plugin 'some_error' from '.': bad\n"
b = ("Unable to load plugin 'some_error' from '.': No module named "
 "'breezy.testingplugins.some_error'\n")

----------------------------------------------------------------------
Ran 2 tests in 0.036s

FAILED (failures=1)