How to reproduce:

1. As a teacher, open an assign.
2. Download it if needed.
3. Go offline and open a submission.
4. Click to edit "Feedback comments".
5. Make some changes, and then save them in offline.
6. After doing this, when you try to leave this submission it will always say there are changes that will be lost, but that's not true.

I think I found how to fix it. In AddonModAssignFeedbackCommentsHandlerService, in 
hasDataChanged, I think initialText should come from offlineData?.plugindata, and if there's no offline data then it should use AddonModAssign.getSubmissionPluginText.
 
However, when looking at this I noticed that the function AddonModAssignFeedbackCommentsHandler.getTextFromInputData sometimes returns an object with text and format, but TS says it will always return a string. We need to review this carefully and update the types so they match the actual values.