Rapid failover/failback problem in AWS

We experienced a network-based failover on our F5 pair in AWS. Both are running 12.1.3.5. The logs from secondary show it detected a connectivity problem to primary and took over:

Jul 19 16:26:49 f5bigip-2 notice sod[6127]: 010c007e:5: Not receiving status updates from peer device /Common/f5bigip-1.mydomain.com (10.1.2.39) (Disconnected).

When this occurred, the primary released its traffic group and AWS moved the floating IP to secondary.

Almost immediately, the primary was detected to be healthy again:

Jul 19 16:26:49 f5bigip-2 notice sod[6127]: 010c007f:5: Receiving status updates from peer device /Common/f5bigip-1.mydomain.com (10.1.2.39) (Online)

This triggered a failback to primary. However, the floating IP stayed on the secondary.

I theorize the sudden failover/failback caused a problem for the "ec2:assignprivateipaddresses" API call to AWS EC2 that is responsible for shifting the floating IP between AWS instances. I have opened case with F5 & AWS for troubleshooting, but just curious if anyone has run in to this before.