partial-dir not being used?!

[Carlos Carvalho]

Here's a running instance caught by ps (lines broken for readibility):

rsync --perms --times --timeout=3600 --stats --no-motd -hh --force
--hard-links -ii --links --partial-dir=.~tmp~ --recursive --exclude
<some excludes> --out-format=<format> --delete-during
--max-delete=75000 --ignore-errors source dest

During download files are being created at
path/.filename.random-string instead of path/.~tmp~/filename. How can
this be? Version is 3.0.6 on linux.