A changeset comment might be terse to write all this:  let's use the Talk page on United States Bicycle Routes wiki.  Briefly, this was tagged ncn, I did some research into NSBs / All-American Roads and because of the (national-level) US Secretary of Transportation's designation of these (where GR "goes first") that ncn is correct.

Then a frequent bike mapper around the Twin Cities changed it back, I contacted him, he saw my point(s) and said, in essence, yes, these are more quasi-national (because of this).

We (OSM-US) remain in a "listening mode" as GR really is "going first" here.  If it continues to vacillate or we don't get more bike routes that are NSBs entered into OSM that are tagged ncn, well, it could go either way.  But for now, it is "emerging" that ncn is correct.