--dir-diff not working with partial path limiter

Hello!

Today I started using --dir-diff and noticed a problem when specifying a
non-full path limiter. My diff tool is setup to be meld (*1).

OK while working directory is repo root; also OK while working directory is
repo subfolder "actual":
git difftool --dir-diff HEAD~1 HEAD -- actual/existing/path
=> meld opens with proper dir-diff.

NOT OK while working directory is repo subfolder "actual":
git difftool --dir-diff HEAD~1 HEAD -- existing/path
=> nothing happens, as if using "non/such/path" as the path limiter.

Because "git diff HEAD~1 HEAD -- existing/path" while the working directory
is the repo subfolder "actual" works, I epxected the difftool to work
similarly. Is this a bug?

Best,
Bernhard

(*1)
[diff]
tool = mydiffmeld
[difftool "mydiffmeld"]
cmd = meld --auto-compare --diff $LOCAL $REMOTE
[difftool]
prompt = false