The simplest tarcopy program seems to result in output that GNU tar, bsdtar, and even Emacs tar-mode is unable to correctly process.
It appears that the resulting tar file is missing files, but examination of the raw output shows they might be there, but just corrupt.
GNU tar actually complains while reading the file.
   https://github.com/mcr/python3-tar-copy-failure

has a test case.  Here is the stupid code to reproduce it:

import tarfile
out = tarfile.open(name="./t2.tar", mode="w", format=tarfile.PAX_FORMAT)
with tarfile.open("./t1.tar") as tar:
    for file in tar.getmembers():
        print (file.name)
        out.addfile(file)
out.close()

This has been confirmed on python 3.6.9 (Ubuntu 18.04 LTS), and python 3.7.3 (Devuan Beowulf).  It seems to omit different files on 32-bit and 64-bit systems.