Is 'api' a reserved directive?

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I don’t know if this is the correct forum, but we can’t use ‘API’ as a restv2 directive. We used to be able to use it when we newly installed webMethods Integration Server 10.1, but after a patch, calls through /API/ just returns a 500 status code. We can use ‘Api’ with a capital ‘A’ just fine if we use that as a restv2 directive, so I’m assuming ‘API’ is reserved? Is there documentation for this?