https://github.com/aws/aws-sdk-cpp/blame/f5309cf6692124b6a20564615a8dab30e974e9d3/src/aws-cpp-sdk-core/source/auth/GeneralHTTPCredentialsProvider.cpp#L36
Looks like the problem is that in case of IPv6 authority part of URI is [fd00:ec2::23], with IPv6 inside [], while in this comparison we are trying to compare it with loopback address fd00:ec2::23, without []. So it leads to an error
Can't use General HTTP Provider: AWS_CONTAINER_CREDENTIALS_FULL_URI is not HTTPS and is not within loop back CIDR: [fd00:ec2::23]
When using with EKS Pod Identity agent, since it's URL (which couldn't be changed with configuration) is:
http://[fd00:ec2::23]/v1/credentials