Why do CloudFormation stack dependencies report UPDATE_COMPLETE in reverse order?

Question

I have multiple CloudFormation stacks with serial dependencies (Stack A2 depends on Stack A1, etc.).
I've noticed a pattern during stack updates where CloudFormation appears to report the UPDATE_COMPLETE status in reverse dependency order.

Looking at the CloudFormation timeline:

1. Parent stack reaches UPDATE_COMPLETE_CLEANUP_IN_PROGRESS
2. Child stacks report UPDATE_COMPLETE in reverse dependency order
3. For example, if A2 depends on A1:
   • A1 updates first (as expected)
   • A2 updates next (as expected)
   • A2 reports UPDATE_COMPLETE first
   • A1 reports UPDATE_COMPLETE last

These are sibling stacks (not nested), with DependsOn between them.

Adding a snippet of Cloudformation's TimeLine view:

The top most bar is the latest stack.
The green blocks are each stack reporting success.