Before 3.1.0, rsync removed any created temp files if interrupted by, e.g., ctrl-C. It does not in 3.1.0. Instead, the temp file is left in the destination directory. It remains even if another rsync is initiated (and if the new rsync is interrupted, another temp file is left).
I could not find an applicable bug when searching, so please excuse this report if one already exists or if this has been fixed. Note I set OS to Linux/x86-64, since that is the only platform I tested.
Thanks for noticing -- I use --partial w/--partial-dir so much that I didn't notice that I broke this recently. It's fixed in git.