Hi Matt,
I'm not sure why you encountered this behavior. I've tested your scenario but I cannot replicate it.
Here's what I've did.
Added a custom action
with sequence and schedule it after the
Finish Execution action group from the
Install Execution Stage (as it says in our
How do I uninstall another MSI package when my application is uninstalled? article ). You can use this image for your reference:

- FinishExecutionStageCA.png (26.4 KiB) Viewed 5560 times
As you can see, the
Condition field is fully enabled. Can you please review your project and see if the behavior still persist?
Best regards,
Dan