Forgerock AM treat's realm's as case insensitive. One can not create /customers realm if /Customers is already created. A call to AM will return results if you use /customers or /Customers, and the XUI will maintain the realm case that you specified when coming to it.
The Web Agent should not force case sensitivity for realm or agent_realm.
If for what ever reason, AM returns in the JWT id_token, the agent_realm in a different case then what the Agent is configured under, it will return an error like this, agent_realm does not have the expected value:
Even though the realm was created as /Customers and the agent is configured with /Customers for the realm, it shouldn't matter the case of the realm or agent_realm that comes back.