Edit Sub-task issue of an Agile Project always detect changes even though no change is made

Cause:
An epic link field in the Edit Screen is causing the popup message to appear even though there is no change made. There's an error message in the screen stated that A subtask cannot be assigned to an epic.

Steps to Reproduce:

1. Create a new "Agile" project
2. Create an issue for the project.
3. Create a sub task for the issue of the project
4. Press "E" or click on Edit
5. Click on Cancel. Popup message stated that "You have entered new data on this page. If you navigate away from this page without first saving your data, the changes will be lost." is displayed.

Expected: No popup appear if there is no change made
Actual Result: Popup message stated that "You have entered new data on this page. If you navigate away from this page without first saving your data, the changes will be lost." is displayed.

Notes
There's an error message in the screen stated that A subtask cannot be assigned to an epic.

Workaround:

1. Create a new screen for sub-task. You could just simply copy the screen from the original agile screen and make sure you remove the epic link field from the newly created screen.
2. Create a new screen scheme and configure the scheme so that the scheme is using the screen that you've just created for the sub-task.
3. Go back to the Agile project and associate the screen scheme to issue type sub-task.