Need clarification on automatic upgrade behavior for MySQL 8.0.42 with RDS Extended Support enabled

My understanding is, the behavior based on your current RDS configuration:

• Failed Manual Upgrade: If the manual upgrade to 8.4.8 fails, the database will roll back to 8.0.42. AWS will not automatically retry the upgrade.
• Automatic Upgrades (8.0.x vs. 8.4.x): AWS will not automatically upgrade your instance to major version 8.4.x. Furthermore, because "Auto Minor Version Upgrade" is disabled, AWS will not routinely upgrade the instance to 8.0.46.
• The "Auto Minor" Exception (Mandatory Upgrades): The only scenario where AWS overrides the disabled "Auto Minor" setting is if your specific minor version (8.0.42) reaches its deprecation date or requires a critical security patch. Only then will AWS force an upgrade to a supported minor version within the 8.0 family.
• Timing: Any automated action by AWS—whether a mandatory engine upgrade, OS patch, or hardware maintenance—will strictly occur during your configured maintenance window.
• Pending Maintenance Flag: This is a generic indicator. It does not exclusively mean an engine upgrade. It frequently indicates underlying OS updates, CA certificate rotations, or hardware maintenance. You need to check the Pending maintenance tab in the RDS Console or the AWS Health Dashboard to identify the exact action.

Note: Once your manual upgrade to 8.4.8 is successful, the instance will return to Standard Support, and Extended Support charges will automatically cease.

see also:

• "In cases of critical security issues or when a version reaches its end-of-support date, Amazon RDS might apply a minor version upgrade even if you haven't enabled the Auto minor version upgrade setting." -> https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_UpgradeDBInstance.Upgrading.html

• https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/extended-support.html

Based on your configuration, here's clarification on the automatic upgrade behavior:

1. If the manual upgrade fails or is rolled back: The instance would remain on its current version (MySQL 8.0.42). AWS does not automatically retry failed manual upgrades.

2. Automatic upgrade behavior with Extended Support: Since you have Auto Minor Version Upgrade disabled, AWS will not automatically upgrade your instance to a newer minor version within the same major version family. However, when a major engine version reaches the end of standard support, RDS automatically upgrades instances enrolled in Extended Support to the last minor version released before the end of standard support date - but this upgrade only occurs after the end of standard support date has passed, not before.

3. Timing of automatic actions: Any automatic upgrades that do occur would take place during your configured maintenance window.

4. Impact of disabling Auto Minor Version Upgrade: Yes, this setting affects automatic minor version upgrades. With this setting disabled, your instance will not receive automatic minor version upgrades during maintenance windows. For automatic minor version upgrades to occur, the Auto Minor Version Upgrade setting must be enabled, and your backup retention period must be greater than 0.

5. Pending Maintenance indication: Pending maintenance can indicate various types of maintenance activities, including but not limited to engine upgrades, system updates, or other required maintenance. You can review the specific details of pending maintenance through the AWS Support Center at https://console.aws.amazon.com/support/ or in your RDS console.

Since you're manually upgrading to MySQL 8.4.8, you'll be moving to a version under standard support, which would end your Extended Support enrollment for this instance.
Sources
Automatic minor version upgrades for RDS for MySQL - Amazon Relational Database Service
Overview of Amazon RDS Extended Support - Amazon Relational Database Service