xn---.example incorrectly decoded as \200.example
With libidn2-2.1.1a-1.fc29.x86_64 in Fedora 29, I see this:
$ echo xn---.example | LC_ALL=en_US idn2 -d | od -c
0000000 200 . e x a m p l e \n
0000012
(Works also in a UTF-8 locale, yielding the U+0080 codepoint.)
I think this is not correct because xn---.example
is an invalid encoding of the empty label, and not of a label with just one character.