Importing Passthrough GraphQL API requires Port 80 on internal vnet apim

Question

I have an APIM instance in internal VNET mode it with an NSG that restricts outbound traffic to the documented list of services required for APIM and also it allows port 443 HTTPS to internet. We do not allow non encrypted http access over the internet on port 80.

When importing a passthrough graphql api into APIM the import will fail with a timeout error.

If I allow port 80 to internet in the NSG then the graphql schema import will succeed.

I am not aware of any requirements of graphql requiring port 80. There is nothing documented that graphql import requires port 80 to open.

Why does APIM need HTTP on port 80 in the case of a graphql api.