I have some Outlook tasks set to (say) "Regenerate 2 weeks after each task is completed". The range of occurrence is from over a year ago, with no end date.

Occasionally when I mark such a task as completed, it doesn't regenerate but just disappears. When it happened today I was able to use ctrl-Z to bring it back (as "not completed"). I tried marking it complete again with the same result, though this time ctrl-Z did not bring it back. It shows up in the list of completed tasks, but not in the not-completed ones.

This is very annoying because if I don't notice that the task has disappeared the date will come when it's supposed to remind me and it doesn't.

Why does this happen and how can I stop it? Thanks.