Yes, it’s easily repeatable. My database has over 4,000 entries – all of which were created without the hours extension. If I ever edit an existing entry, without touching the hours section, it will add the hours section showing all days as closed.
Also, by default, all new entries show all days as closed unless you put hours in. I’m assuming this is because you use blank entries as CLOSED. A better method would probably be to require you to actively specify closed days, and blank would leave the entry disabled.
