The real question
It's been asked several times here, but I see no attempts at an authoritative answer to:
"Is ipv6_1-cxl0-c088.1.lhr004.ix.nflxvideo.net a host name?"
According to @jake's own referenced source, host names are name fields of A or MX records or the data fields of the SOA and NS records. This makes sense to me and seems to jibe with all the RFCs. The dig utility shows me that ipv6_1-cxl0-c088.1.lhr004.ix.nflxvideo.net is the name field of an A record. (I didn't pull their zone data because people in my country have been prosecuted for that.) Underscores are not allowed in host names, so I don't see why this, as @jake put it, "is OK the way that Netflix is using it".
Maybe the library is removing the underscores for the wrong/no reason(s) and still needs to be patched, but that doesn't make the Netflix host name valid.
I'm interested in what, if anything other than the RFC's terms, are broken my host names with underscores. None of my resolvers seem to care, perhaps because they are forgiving and trying not to make a distinction when the impact is nil (except to RFC writers).