Its not the lambda run time that gets Ipv6, its the lambda API that is now dual stack. (the endpoint you hit when you want to invoke a lambda.)

When you do an invoke, if you use lambda.us-east-1.api.aws instead of the old ipv4 only lambda.us-east-2.amazonaws.com you will hit a v6 ula if you are on v6:

so if you want to talk to ipv6 unicast endpoint to invoke a lambda do so by specifying the new endpoint with --endpoint-url:

aws lambda invoke --function-name my-function out --log-type Tail --endpoint-url lambda.us-east-1.api.aws

> lambda.us-east-1.api.aws Server: one.one.one.one Address: 1.1.1.1

Non-authoritative answer: Name: lambda.us-east-1.api.aws Addresses: 2600:1f18:20cb:b301:7ced:3d08:1e2b:ed32 2600:1f18:20cb:b303:fa58:3743:b0dd:f703 2600:1f18:20cb:b302:d19a:21c7:11ee:8ad2 54.85.112.20 3.220.153.240 34.235.81.32