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When adding the fix for #20160 I had added a test involving passing a struct by value to a Python callback. I modified that test to update the struct in the callback, and check that the passed-in struct hadn't changed.
 def test_callback_large_struct(self):
 class Check: pass
 class X(Structure):
 _fields_ = [
 ('first', c_ulong),
 ('second', c_ulong),
 ('third', c_ulong),
 ]
 def callback(check, s):
 check.first = s.first
 check.second = s.second
 check.third = s.third
 # See issue #29565.
 # The structure should be passed by value, so
 # any changes to it should not be reflected in
 # the value passed
 s.first = s.second = s.third = 0x0badf00d
 check = Check()
 s = X()
 s.first = 0xdeadbeef
 s.second = 0xcafebabe
 s.third = 0x0bad1dea
 CALLBACK = CFUNCTYPE(None, X)
 dll = CDLL(_ctypes_test.__file__)
 func = dll._testfunc_cbk_large_struct
 func.argtypes = (X, CALLBACK)
 func.restype = None
 # the function just calls the callback with the passed structure
 func(s, CALLBACK(functools.partial(callback, check)))
 self.assertEqual(check.first, s.first)
 self.assertEqual(check.second, s.second)
 self.assertEqual(check.third, s.third)
 self.assertEqual(check.first, 0xdeadbeef)
 self.assertEqual(check.second, 0xcafebabe)
 self.assertEqual(check.third, 0x0bad1dea)
 # See issue #29565.
 # Ensure that the original struct is unchanged.
 self.assertEqual(s.first, check.first)
 self.assertEqual(s.second, check.second)
 self.assertEqual(s.third, check.third)
The changed test still passes, so it looks like ctypes makes a copy when passing from C to a Python callback, but not when passing from Python to C.