There is a problem with the glibc-2.26-54 updates that prevent IPv6 entries from working in /etc/hosts
Starting with a Amazon Linux 2 AMI with glibc-2.26-48 (amazon/amzn2-ami-hvm-2.0.20210813.1-arm64-gp2)
I can have an IPv6 entry in /etc/hosts at it would work
2001:4860:4860::8888 dns6
$ curl -v6 http://dns6/
- Trying 2001:4860:4860::8888:80...
However, after applying the glibc-2.26-54 updates via sudo yum update
$ curl -v6 http://dns6/
- Could not resolve host: dns6
If I downgrade just the glibc updates
$ sudo yum downgrade glibc-2.26-48.amzn2.aarch64 glibc-all-langpacks-2.26-48.amzn2.aarch64 glibc-common-2.26-48.amzn2.aarch64 glibc-locale-source-2.26-48.amzn2.aarch64 glibc-minimal-langpack-2.26-48.amzn2.aarch64 libcrypt-2.26-48.amzn2.aarch64
$ curl -v6 http://dns6/
- Trying 2001:4860:4860::8888:80...