regression when metadata differ when comparing byte-by-byte identical files

When writing autopkgtests for rsync, I've discovered that diffoscope 240 from Debian stable behaves differently from the version in Debian unstable (297).

When comparing byte-by-byte identical files with different metadata with the following executable shell script:

#!/bin/sh

DIFF_CMD="~/programming/debian/diffoscope/bin/diffoscope --no-progress"  # adapt to your local diffoscope binary
RSYNC_BIN="/usr/bin/rsync"

echo foo > src
cp src target
touch -d "next minute" target

$RSYNC_BIN -tu src target

stat src
stat target
$DIFF_CMD --exclude-directory-metadata=no src target 2>&1
status=$?
echo $status

diffoscope 240 exits with 1 (as expected since the mtimes are clearly different) whereas diffoscope 297 surprisingly exits with 0.

Running git bisect shows that this commit introduces the regression.