Given the inconsistencies, it would be better to disable the button when not expected to work.
Steps to Reproduce
1. Create an Update Set and make it the default.
2. Create an app and a table.
3. Set the update set to Complete.
4. Back out the update set.
Result: The update set appears.
Using team development to push the changes to the parent is another, perhaps more common, scenario where a local update set is completed and cannot always be fully backed out.
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Related Problem: PRB598943