From `grep(1p)`:

  The pattern_list's value shall consist of one or more patterns separated by <newline> characters;

Consider the following invocation:

  printf "foo\nbar\nbaz\n" | grep -Fv -e "$(printf "foo\nbar\n")"

In accordance with the POSIX man page this should output the following:

  baz

This is what GNU grep (for instance) outputs. The busybox implementation outputs:

  foo
  bar
  baz

Instead. Looking at the implementation this is due to the fact that busybox does not check for newline in patterns when the -e option is given.