In recent versions of npm (I'm using 6.9.0) the authentication token that is saved to the .npmrc file is incompatible with nexus, which makes the whole npm adduser command redundant.
so, if I do:
I get this added to my .npmrc
After this, any step that I perform fails with a HTTP 401, including the same adduser command or any install command.
The only way to correctly authenticate is the _auth base64 token detailed in https://support.sonatype.com/hc/en-us/articles/115015110067-Using-User-Token-s-with-NPM
Effectively, by not supporting these type of tokens, I believe that the bugfix from
NEXUS-7989 has regressed.