Global Accelerator - Cannot Upgrade to Dual Stack

When attempting to upgrade our existing accelerator to dual stack we get the following error:

Couldn't update the accelerator to DUAL_STACK because 4 endpoints don't support IPV6. 
Add IPV6 support or remove the endpoints, and try again. 
EndpointARN: arn:aws:ec2:ap-southeast-1:872820258610:instance/i-03569e27eebc8dda9 
EndpointARN: arn:aws:ec2:eu-west-2:872820258610:instance/i-095c957c5134fad32 
EndpointARN: arn:aws:ec2:us-east-2:872820258610:instance/i-08a2e3d77cd1d2e85 
EndpointARN: arn:aws:ec2:us-west-1:872820258610:instance/i-043f544ccd6ead5eb 

All of the endpoints have globally routable IPv6 addresses. We've verified that all of them can be accesses via IPv6 from hosts on the internet.

What could be causing the accelerator to declare the endpoints do not support IPv6?