Unable to display APIs in Azure Function App Swagger/OpenAPI

**Describe the issue**
We have an Azure Func app hosted in **Linux Premium plan, private network, using only https**, but the Swagger UI cannot display APIs like below:

[![enter image description here][1]][1]

The settings below are set based on the link below, but the error above still occurs.

{
"Values": {
"OpenApi__ForceHttps": "true",
"OpenApi__ForceHttp": "false",
"OpenApi__HostNames": "/api"
}
}

https://github.com/Azure/azure-functions-openapi-extension/blob/main/docs/openapi.md

**Settings**

.NET Core 3.x

Microsoft.Azure.WebJobs.Extensions.OpenApi 1.0

**Refs:**

https://github.com/Azure/azure-functions-openapi-extension/blob/main/docs/openapi-core.md
https://github.com/Azure/Azure-Functions/issues/1933
https://github.com/Azure/azure-functions-openapi-extension/issues/352

[1]: https://i.stack.imgur.com/enaPP.png