I reproduced this in python 3.8 and python 3.6. The last line displays the bug itself, all other lines do the setup and pretty much explain WHY I think that is the bug.

class Class:
    def method(self): ...

instance = Class()

# expected: ids match
assert id(Class.method.__get__(None, Class)) == id(Class.method)
# expected: __eq__ returns True
assert Class.method.__get__(None, Class) == Class.method
# expected: is returns True
assert Class.method.__get__(None, Class) is Class.method

# expected: ids match
assert id(Class.method.__get__(instance, Class)) == id(instance.method)
# expected: __eq__ returns True
assert Class.method.__get__(instance, Class) == instance.method
# UNEXPECTED: is returns False, why?..
assert Class.method.__get__(instance, Class) is not instance.method  # why?

This is not a bug. The "is" operator works as expected. A method descriptor returns a new wrapper object on each access.

CPython uses free lists to "recycle" memory locations to increase performance. id(Class.method.__get__(None, Class)) == id(Class.method) is true because the return value of "Class.method.__get__(None, Class)" is garbage collected and the memory address is reused.

See:

>>> class Class:
...     def method(self): ...
... 
>>> instance = Class()
>>> m1 = Class.method.__get__(instance, Class)
>>> m2 = instance.method
>>> id(m1) == id(m2)
False

@christian.heimes thanks for the feedback