rsync - Nov 2020 - Destination file lost with --link-dest and --backup-dir options

Hello,

I am facing an issue when using --link-dest and --backup-dir together. When a
source file differs from the file in the destination directory and the --backup
and --backup-dir options are provided then a copy of the destination file is
made in --backup-dir. However, if additionally --link-dest is provided and the
source file is found in --link-dest then the file in the destination is not
copied to --backup-dir but deleted instead. The following commands demonstrate
the issue:

> mkdir src ref tgt bkp
> touch src/file ref/file
>
> sleep 1
>
> echo "content" > tgt/file
>
> echo "Initial stats: stat -c '%i %n %y'" src/* ref/*
tgt/* bkp/*
> stat -c '%i %n %y' src/* ref/* tgt/* bkp/*
>
> echo
> echo "Running: rsync --link-dest=../ref -a -b --backup-dir=../bkp src/
tgt"
> rsync --link-dest=../ref -a -b --backup-dir=../bkp src/ tgt
>
> echo
> echo "Final stats: stat -c '%i %n %y'" src/* ref/* tgt/*
bkp/*
> stat -c '%i %n %y' src/* ref/* tgt/* bkp/*
> echo "File in 'tgt' is hardlinked against 'ref' but no
backup of previous file in 'bkp'."

I checked the manpage entries for --link-dest and --backup-dir, none of them
explain the observed behaviour. While the passage at --link-dest warns that
destination files may get their attributes "tweaked", it doesnot
explicitly state that the --backup and --backup-dir options will be ignored.