When either file's last line has no newline, diff -e prints nothing on stdout:

$ printf 'a\nb' > x; printf 'a\nc\n' > y
$ diffutils diff -e x y
No newline at end of file
$ echo $?
2
$ /usr/bin/diff -e x y    # GNU diff 3.12
2c
c
.
/usr/bin/diff: x: No newline at end of file

$ echo $?
2

GNU still writes the ed script, then warns on stderr for each file that lacks the final newline (diff: FILE: No newline at end of file, then a blank line) and exits 2. Both exit 2, but a caller that applies the script, or shows it, gets nothing from uutils.

The early return is in src/ed_diff.rs:65 and src/ed_diff.rs:71 (DiffError::MissingNL). The test missing_newline (tests/by-util/test_diff.rs:122) checks for the current stderr (starts_with("No newline at end of file")), so matching GNU means changing that test. I have a change that matches GNU for either file, both, two different files with the same content, an empty file and stdin (apart from 2,2d, which is #308); happy to open a PR if you want GNU's output here.

uutils diffutils main 8ff5bda and v0.6.0.