Board Thread:The Quarters - Q & A/@comment-219.89.192.44-20140307021825/@comment-24150802-20140307024104

Yes, that's expected behaviour. HabitRPG removes to-dos three days after they were completed to prevent the webpage containing your tasks from becoming very slow to load and use.

The "User Data" export option includes to-dos, but I can't recall if old ones are included. I suspect not, but it might be worth you checking.