Unable to display APIs in Azure Function App Swagger/OpenAPI

[PingPongSet]

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:

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
Azure/Azure-Functions#1933
#352