Upcoming OS upgrade for RDS mysql instances - Mandatory?

We are currently on mysql 8.0.16 on a t2.2xlarge instance. We have automatic updated turned OFF. According to the documentation here: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_UpgradeDBInstance.Maintenance.html#Mandatory_OS_Updates we are not affected by this. however.... We have tried creating a new replica on the new minimum required mysql version (8.0.23). We are in a situation where replication is broken due to collation problems (even through the replica is based on a fresh snapshot of our primary). It appears to be a problem with the scenario: primary (8.0.16) -> replica (>= 8.0.23)

We are now worried that we will be forcibly upgrades (OS and mysql version) next month, leaving us in a broken state. Can anyone shed some light on this and what course of action we must take? Many thanks.