This looks like a simple reference cycle, as far as I can tell; I don't see an actual refleak.  Reproducing using Mark's code from StackOverflow (adjusted):

$ ./python.exe -X showrefcount
Python 3.9.0a6+ (heads/master-dirty:d10091aa17, May  8 2020, 17:43:51) 
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 
[48551 refs, 15489 blocks]
>>> from ctypes import *
[53774 refs, 17227 blocks]
>>> s = create_string_buffer(1<<30);del s
[53832 refs, 17254 blocks]
>>> s = create_string_buffer(1<<30);del s
[53832 refs, 17255 blocks]
>>> s = create_string_buffer(1<<30);del s
[53832 refs, 17254 blocks]
>>> s = create_string_buffer(1<<30);s = cast(s, c_void_p);del s
[53839 refs, 17258 blocks]
>>> s = create_string_buffer(1<<30);s = cast(s, c_void_p);del s
[53844 refs, 17262 blocks]
>>> import gc
[54078 refs, 17315 blocks]
>>> gc.collect()
13
[54030 refs, 17182 blocks]
>>> 
[54030 refs, 17199 blocks]
>>> s = create_string_buffer(1<<30);del s
[54077 refs, 17227 blocks]
>>> gc.collect()
9
[54030 refs, 17182 blocks]
>>> s = create_string_buffer(1<<30);s = cast(s, c_void_p);del s;gc.collect()
11
[54030 refs, 17182 blocks]
>>> s = create_string_buffer(1<<30);s = cast(s, c_void_p);del s;gc.collect()
11
[54030 refs, 17182 blocks]



I'll freely admit that I'm way out of my depth here, but this doesn't look like a bug as far as I can tell.  `ctypes` seems to be creating some cyclic references internally, but a collection clears them right up.