The "using an external DS repository" procedure uses the default userRoot backend created with the setup command. This creates an equality index for "uid" automatically. If we create a new backend for IDM repo after DS setup, then the "uid" index is not included by default. https://backstage.forgerock.com/docs/ds/6/admin-guide/index.html#default-indexes
The installation guide should be explicit about which indexes are needed by IDM repo, and which are created by default.