Facing issues after upgrading EKS from v1.25 to v1.26

Once you upgrade a cluster, you can't downgrade to a previous version.

The only way to get to a previous version would be to deploy a new cluster. It may be worth in the future to instead of upgrading the cluster, create a 2nd cluster and move your pods across in a phased migration/upgrade in case you have any of these issues again.

https://docs.aws.amazon.com/eks/latest/userguide/update-cluster.html

Similar issue here https://learn.microsoft.com/en-us/troubleshoot/azure/azure-kubernetes/aks-memory-saturation-after-upgrade

You might look if you could you be impacted by the rollout of CgroupsV2. The standard AL2023-based EKS-optimized AMI uses Control Group version 2 (cgroup v2). Old JAVA runtimes are not cgroupv2 aware and can not autodetect container memory limits ... leading to OOMs. Just in case.