_ disallowed in HostName

https://bugs.openjdk.java.net/browse/JDK-8170265

https://tools.ietf.org/html/rfc952

   A "name" (Net, Host, Gateway, or Domain name) is a text string up
   to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
   sign (-), and period (.).

http://ietf.org/rfc/rfc2782.txt
        The symbolic name of the desired protocol, with an underscore
        (_) prepended to prevent collisions with DNS labels that occur
        in nature.  _TCP and _UDP are at present the most useful values
        for this field, though any name defined by Assigned Numbers or
        locally may be used (as for Service).  The Proto is case
        insensitive.
http://domainkeys.sourceforge.net/underscore.html

Host names are not allowed to have underscores in them. In DNS, host names are the name fields of A or MX records or the data fields of the SOA and NS records. Thus, there are many DNS entries that are not hostnames.

Thus, the "_" is purposedly used to disambiguate hosts from other attributes in the DNS. In fact that's the very reason that the "_" prefix was chosen to differentiate selectors and to avoid collisions with existing namespace.
Similarly, O'Reilly's DNS and Bind states in Chapter 4, section 5, 'Names that are not host names can consist of any printable ASCII character.'​