All good questions!
We welcome people to transcribe without an account in order to make the site accessible and inviting to all. We want people to easily be able to try out the site--with the hope that they will enjoy the project and then decide to register. Registered users do get the additional features and benefits of being able to, tag, review and track their work.
I wasn't able to replicate this issue, but please let us know if it continues to happen. In the meantime, the best solution is probably closing your browser tab or restarting the browser. So putting your work in a text document is a good idea to prevent loss of work.
I found that the site logged me out while i had my newspaper transcription open overnight.
Luckily I didn't lose anything, and I know now to keep track of my log-in status.