In the procedure for Add Existing MongoDB Processes to Monitoring, is says:

"If you add processes from an environment that uses authentication, you must add each mongod instance separately and explicitly set the authentication credentials on each."

1. This goes against the UI, which gives the user the ability to choose 'replica set' and add credentials for the set
2. I asked some of the MMS engineers about this and was told that RS members with auth in use should indeed be added individually, but when I tried adding the 3 nodes as standalone nodes, it didn't work well – I ended up with a RS containing the 3 nodes plus a duplicate of one of the nodes as a standalone. When I import the RS as a RS, giving the primary, I get the desired result.

If this directive is truly correct, then a bug needs to opened for the UI to enforce it.