eventually we'd get one with the same Mac address but new internal IP address
This sounds like an EC2 bug, not an ElastiCache bug. EC2 uses "locally administered addresses" for MACs (as it should), but the administrator assigning MAC addresses is responsible for ensuring that addresses are not reused within a collision domain.
This sounds like an EC2 bug, not an ElastiCache bug. EC2 uses "locally administered addresses" for MACs (as it should), but the administrator assigning MAC addresses is responsible for ensuring that addresses are not reused within a collision domain.