In the posting window there is an option to "Save draft". The user can then continue to work on the post via UCP->Manage drafts.
If the user selects "Load draft", the post looks normal, but if the user selects "View/Edit", the post text contains HTML characters. This appears to be a change from the phpBB 3.1.x experience, so is perhaps related to the new BBCode engine?
See https://www.phpbb.com/community/viewtopic.php?f=556&t=2434981 for more details and an example.