I've run into this a few times with the Google Ads API. According to the documentation, the enum values for the WebpageConditionOperand are "UNSPECIFIED", "UNKNOWN", "URL", "CATEGORY", "PAGE_TITLE" , "PAGE_CONTENT", and "CUSTOM_LABEL" in all caps with underscores exactly as they were in the AdWords API.
However, when using those enum values, an error is thrown. In investigating, I notice that drilling down through the enum in Visual Studio, the actual enum values are "Unspecified", "Unknown", "Url", "Category", "PageTitle", "PageContent", and "CustomLabel" with a mix of upper and lower case and with no underscores.
I'm finding it really difficult to code for values where the documentation does not match the API.