If you need to get the assignment back to play some more, you can add /undelete to the end of your main course URL. Example: /courses/12345/undelete
Do you feel comfortable using the HTML editor? It gives you ultimate control over what you can and cannot do.
This is a long shot, but what browser are you using? Have you tried editing the page with a different browser? Chrome, Firefox, and Safari play the nicest, but there have even been issues with people using older versions of those browsers.
What you're describing is definitely not the expected behavior, so you may need to end up contacting your Canvas support as you mentioned earlier.
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