Can't establish connection with custom domain - websocket - api gateway

So, I have setup a custom domain for my REST and Websocket apis in AWS API Gateway. Now, I can connect on all of my custom REST endpoints without any problems but the websocket custom domain doesn't seem to work only with the custom domain in place (but the auto generated endpoint just works)

wss://vxxxxx16.execute-api.eu-central-1.amazonaws.com/dev

wss://ws.mydomain.app/dev-base

There is not much info present in the Dev tools, please see below.

For successful connection :

For failed connection :

I am confused, where I am missing the logical step here. The setup in the aws is like the following :

1. Regional certificate is issued for websocket custom endpoint
2. A, AAAA Records are added in Route53
3. Base domain mapping is correctly set for custom domain

What else could be needed ?