underscores get stripped

I'm using idn2_lookup_u8(..., IDN2_NFC_INPUT | IDN2_NONTRANSITIONAL). Initial underscores in labels get stripped. It this expected? Example:

"_443._tcp.fedoraproject.org" → "443.tcp.fedoraproject.org"

I'm using libidn2-2.0.2-1.fc26.x86_64.