Error Message :
N/A

Adding a variable to an instance of an object remains if the variable is modified within the "Properties" window and then the variable is deleted.

Investigation

To replicate this behavior:

1.  Add a variable on the Variables & Prompts tab of a workflow and give it a default value, then save and close the workflow. 
2. Add the workflow as a task of a master workflow. 
3. Open the tasks properties of the sub-workflow and modify the value for the variable (from step#1) on the Variables & Prompts, and save and close the master workflow. 
4. Open the sub workflow object again., delete the variable (which was added in step #1) from the Variables & Prompts tab, then save and close. 
5. Open the master workflow again, look at the task properties of the sub-workflow and view its Variables & Prompts. The variable is still present even though it had been deleted from the sub-workflow's object definition. 

Cause type:
By design
Root Cause: There is no connection between the instance of an object and the core definition.

OS Version: N/a

This behavior is as designed. 

Use a PRPT instead of directly adding to variables within the object itself. These will be removed properly on deletion. 

Fix Status: No Fix

Fix Version(s):
N/A

Workaround :
N/A