With the following makefile:

%foo:
echo $(@D)

make responds:

$> make obj\foo
echo obj\
obj\
$> make obj/foo
echo obj
obj

The documentation states that "the ‘D’ variants all omit the trailing slash which always appears in the output of the dir function." Technically a backslash is not a slash, but the intention here was to remove the trailing path separator. Therefore I recommend that Windows version removes both separators in order to keep the behavior consistent.

The issue is also reproducible on 3.82.