Can't update custom domain for Developer Portal

Question

I've created an APIM with APIs. I want to use my domain name to override the default *.azure-api.net for both api and developer portal endpoints.

Following the instruction, I added 2 gateways in custom domains. One is api.mydomain.com, one is portal.mydomain.com.

I then added 2 CNAME in my domain DNS records, the 2 above records point to myapim.azure-api.net and myapim.developer.azure-api.net respectively.

After the records finished propagating, where I confirmed by using DNS tool. I found that the Gateway URL in my APIM did update to api.mydomain.com. I can use the original portal URL and can see the API URL has been updated to "api.mydomain.com/v1/test/operation1" and they work very well.

However, the Developer portal URL stays unchanged -- it's still myapim.developer.azure-api.net.

When I browse [https://portal.mydomain.com], it shows { "statusCode": 404, "message": "Resource not found" }

I then tried to republish the developer portal but everything stays same.

Is there anything I missed?

Thanks for your help.