This will elide all naming information. See:

>>> os.fspath(m.absolute)
'MagicMock/140083495922656'
>>> str(m.absolute)
"<MagicMock name='mock.absolute' id='140083495922656'>"

Or when a mock is given a name. The id is unique per attribute and probably quite unuseful for the user when debugging issues.

Have you considered:
lambda self: f"{self._extract_mock_name()}/{id(self)}"

It would look nicer in some scenarios like:

>>> os.fspath(mock.MagicMock(**{"__fspath__.return_value": "abc"}).absolute)
'mock.absolute/140476946856592'

But maybe weird for some others like:

>>> os.fspath(mock.MagicMock() / 'other' / 'path')
'mock.__truediv__().__truediv__()/140476946879824'

Agree that naming information should be preserved, what would you think of:

lambda self: f"{type(self).__name__}/{self._extract_mock_name()}/{id(self)}"

The __truediv__ case is a bit strange but it is correct, though.