The default write concern for some user & role management methods are majority. This was not mentioned in the docs, which leads to some confusion from users, since the default write concern for a replica set is w:1

This involves these user management methods:

• createUser
• updateUser
• dropUser
• dropAllUsers
• grantRolesToUser
• revokeRolesFromUser

And these role management methods:

• createRole
• updateRole
• dropRole
• dropAllRoles
• grantRolesToRole
• revokeRolesFromRole
• grantPrivilegesToRole
• revokePrivilegesFromRole

The default write concern of majority for these methods should be explicitly mentioned in the documentation, since it deviates from expected standard behaviour.

Appendix:

• The relevant section in the 3.6 codebase: link
• The original ticket for adding write concern to these methods: SERVER-6246