Looks like the logs got dropped...Anyways, I did more testing (Alpine 3.6, Guacamole 0.9.13) and narrowed it down to the "user-mapping" property contained within the guacamole.properties file not being read.
guac:/etc/guacamole# cat guacamole.properties
Using the default pathing for user-mapping.xml, I'm able to succesfully authenticate
13:37:49.227 [http-nio-8080-exec-6] DEBUG o.a.g.a.f.FileAuthenticationProvider - Reading user mapping file: "/etc/guacamole/user-mapping.xml"
13:37:49.245 [http-nio-8080-exec-6] DEBUG o.a.g.r.auth.AuthenticationService - Anonymous authentication attempt from 192.168.50.100 failed.
13:38:00.100 [http-nio-8080-exec-8] INFO o.a.g.r.auth.AuthenticationService - User "username" successfully authenticated from 192.168.50.100.
13:38:00.105 [http-nio-8080-exec-8] DEBUG o.a.g.r.auth.AuthenticationService - Login was successful for user "username".
13:38:00.432 [http-nio-8080-exec-7] DEBUG o.a.g.net.InetGuacamoleSocket - Connecting to guacd at localhost:7777.
But moving the default user-mapping.xml to another location results in the following log entry and updating guacamole.properties accordingly yields::
13:47:53.141 [http-nio-8080-exec-4] DEBUG o.a.g.a.f.FileAuthenticationProvider - User mapping file "/etc/guacamole/user-mapping.xml" does not exist and will not be read.
Is there something I'm overlooking that's really simple? I can confirm that the guacamole.properties file is being read since I can change both the server listening port (guacd via guacd.conf) and client communications port (via guacamole.properties) to one other than 4822.