While working on the openSUSE mirror infra, I found that
with rsync-3.2.3-150400.3.3.1.x86_64
rsync --delay-updates never updates files after interruption

Steps to reproduce:

#!/bin/sh
rm -rf src dst
mkdir -p src/ dst/.~tmp~
echo x > src/x
echo x > dst/.~tmp~/x # simulate failed previous rsync
o="-av --delay-updates"
rsync $o src/ dst/
rsync $o src/ dst/
ls -l dst/

Actual result:
every rsync call syncs x again and ls -l shows an empty dir 
And .~tmp~ is still there

Expected result:
rsync --delay-updates should move synced files from .~tmp~ after sync is done.