The logic for bailing out to a slow copy is currently:

https://github.com/python/cpython/blob/main/Lib/shutil.py#L158

that condition appears to not be happening in Alexei's test.  Suggesting that either at least one sendfile call succeeded and thus offset is non-zero or the lseek failed.

run that test under pdb and walk thru the code, or under strace to look at the syscalls and find out.

The question seems to be is if it should be okay to _GiveUpOnFastCopy after a partial (incomplete) copy has already occurred via sendfile.