split: -C with large argument splits at the wrong place

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit dddbc17), gnu coreutils version 9.5.218-7e5b6

Steps to reproduce:

# One very long line, one very short line, one very long line.
printf '%131070s\n' '' > expaa
printf 'x\n' > expab
printf '%131071s\n' '' > expac
cat expaa expab expac > bigin
split -C 131072 bigin

What happens now: uutils splits at the wrong place:

$ wc -c xa?
131071 xaa
131072 xab
     2 xac
262145 total

What I expected to happen: GNU split splits as expected (after each newline):

$ wc -c xa?
131071 xaa
     2 xab
131072 xac
262145 total

Notes: this is causing a failure in the GNU test file tests/split/line-bytes.sh.