On Fedora32/PPC64LE (5.7.9-200.fc32.ppc64le), with little change:
  libc = CDLL('/usr/lib64/libc.so.6')
I get the correct answer:
b'def'
b'def'
b'def'
# python3 --version
Python 3.8.3
libffi : 3.1-24


On Fedora32/x86_64 (5.7.9-200.fc32.x86_64), with a little change:
  libc = CDLL('/usr/lib64/libc-2.31.so')
that crashes:
b'def'
Segmentation fault (core dumped)
# python3 --version
Python 3.8.3
libffi : 3.1-24


AIX : libffi-3.2.1
On AIX 7.2, with Python 3.8.5 compiled with XLC v13, in 64bit:
b'def'
b'def'
None
On AIX 7.2, with Python 3.8.5 compiled with GCC 8.4, in 64bit:
b'def'
b'def'
None

On AIX 7.2, with Python 3.8.5 compiled with XLC v13, in 32bit:
  ( libc = CDLL('libc.a(shr.o)') )
b'def'
b'def'
b'def'
On AIX 7.2, with Python 3.8.5 compiled with GCC 8.4, in 32bit:
b'def'
b'def'
b'def'

Preliminary conclusions:
 - this is a 64bit issue on AIX and it is independent of the compiler
 - it is worse on Fedora/x86_64
 - it works perfectly on Fedora/PPC64LE
what a mess.