Major - P3 We have a tutorial section describing the process the driver follows to determine auth source (https://docs.mongodb.com/ruby-driver/master/tutorials/ruby-driver-authentication/#auth-source). This section is missing an important aspect of the process - if the client is instantiated with a SRV URI, the driver will query TXT records for this URI for URI options and would merge those with explicitly specified options. Atlas in particular gives authSource=admin in the relevant TXT records which affects the auth source used by the driver.
Example from my test deployment:
butler% dig -t TXT cluster0-tut4d.mongodb.net
; <<>> DiG 9.11.5-P4-5.1-Debian <<>> -t TXT cluster0-tut4d.mongodb.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63677
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 8
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 31d32c93b1d73a67115b65bb5ef61aed183a589080010dd7 (good)
;; QUESTION SECTION:
;cluster0-tut4d.mongodb.net. IN TXT
;; ANSWER SECTION:
cluster0-tut4d.mongodb.net. 60 IN TXT "authSource=admin&replicaSet=Cluster0-shard-0"
;; AUTHORITY SECTION:
mongodb.net. 82425 IN NS ns-1043.awsdns-02.org.
mongodb.net. 82425 IN NS ns-1647.awsdns-13.co.uk.
mongodb.net. 82425 IN NS ns-430.awsdns-53.com.
mongodb.net. 82425 IN NS ns-761.awsdns-31.net.
;; ADDITIONAL SECTION:
ns-430.awsdns-53.com. 82425 IN A 205.251.193.174
ns-761.awsdns-31.net. 82425 IN A 205.251.194.249
ns-1043.awsdns-02.org. 82425 IN A 205.251.196.19
ns-1647.awsdns-13.co.uk. 82425 IN A 205.251.198.111
ns-430.awsdns-53.com. 82425 IN AAAA 2600:9000:5301:ae00::1
ns-1043.awsdns-02.org. 82425 IN AAAA 2600:9000:5304:1300::1
ns-1647.awsdns-13.co.uk. 82425 IN AAAA 2600:9000:5306:6f00::1
;; Query time: 12 msec
;; SERVER: 10.0.9.254#53(10.0.9.254)
;; WHEN: Fri Jun 26 11:59:08 EDT 2020
;; MSG SIZE rcvd: 425