I was able to reproduce this error. I have created a report and assigned it to our forum specialist. Please let me know if you encounter any additional issues with the forums or answerhub.
I have talked about this to our forum crew and title editing is behaving as expected. Unfortunately, on these forums your thread topic is tied to the URL and can cause issues if changed, so it had been restricted. You can still change the threads sub-title, which is the βTitleβ field that you were seeing before. I hope this helps you with your thread for your tutorials.