There is simply no answer to your question.
I have created simple workflow with just two steps:
Update list item. I have played with it for hour or so doing everything I could think of (renaming and deleting fields, changing task and item during workflow, setting different permissions etc). I was unable to reproduce behavior you are experiencing. My workflow ended either with error or with status completed.
My little experiment leads only to one conclusion: Your problem is too localized and you are probably using much more complex workflow then my simple 2-step. But even if you provide more info I doubt that anyone can easily offer some real solution.
So what can you do?
- Republish your workflow and check it again
- Add Log to history list activity after each step (just log 'Step1', 'Step2' ...) and then look at history to determine exact place where your workflow is stuck
- Try to remove all additional activities (one by one) you may have between CDFU and Update and then test workflow again
- Remove all other customizations you may have on list (eg List Item Event Receivers) and test your workflow ... again
- Publish your workflow on different list or better on different site and test it there
- Build simple 2-step workflow (CDFU - Update) and test it on original list
- Completely recreate your workflow under different name and test it on original list
And after you try all of the above post your findings so maybe someone can help you.
Edited nothing: request is in comments.