Interesting, I am infact using v7.13.2 on my end (my bad, I just mentioned the minor version earlier). Interesting that when you tried to connect - you don't see the bucket names prefix. Did you had to enable any additional permissions listed out at the time of plugin installation?
bin]$ ./elasticsearch-plugin install repository-s3
-> Installing repository-s3
-> Downloading repository-s3 from elastic
[=================================================] 100%
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@ WARNING: plugin requires additional permissions @
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* java.lang.RuntimePermission accessDeclaredMembers
* java.lang.RuntimePermission getClassLoader
* java.lang.reflect.ReflectPermission suppressAccessChecks
* java.net.SocketPermission * connect,resolve
* java.util.PropertyPermission es.allow_insecure_settings read,write
See http://docs.oracle.com/javase/8/docs/technotes/guides/security/permissions.html
for descriptions of what these permissions allow and the associated risks.
Continue with installation? [y/N]y
-> Installed repository-s3
-> Please restart Elasticsearch to activate any plugins installed