This functionality regressed with the adjustments
in commit v8.25-4-g62e7af032
* src/split.c (bytes_chunk_extract): Account for already read data
when seeking into the file.
* tests/split/b-chunk.sh: Use the hidden ---io-blksize option,
to test this functionality.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/46048